Installing decorative polystyrene cornices: step-by-step guide

There are two types of people: those who believe installing decorative polystyrene coves is simply "gluing a strip to the wall, what’s so hard about it," and those who have already tried it. The first live in blissful ignorance. The second know the truth: between "glued poorly" and "professionally installed" lies a chasm of dozens of nuances, each affecting the final result. A poorly cut corner won’t align — a gap of a centimeter. Insufficient adhesive — the cove will detach within a week. Forgetting to mark — the strip will be uneven, and this is visible from the threshold.

Professional finishers know: installing a cove is not intuition or luck, but a clear algorithm of sequential operations, each performed according to specific rules. Skipping the base preparation stage — you’ll get peeling. Not checking the horizontal alignment — you’ll get a crooked line that cuts the eye. Applying adhesive incorrectly — you’ll get gaps and sagging. Not properly plastering joints — you’ll get visible seams that ruin the entire picture.

This article is an exhaustive installation manual from the first to the last step. There are no lyrical digressions like "and coves can also be beautiful." Only specific instructions: what to do, how to do it, why exactly this way, which tools to use, how long it takes, what mistakes are possible, and how to avoid them. Following this guide, even someone without experience in finishing work will install the cove at a professional level. Skip even one point — and the result will be at risk.

Go to Catalog

Base Preparation: The Foundation of Success

The quality of installation depends 70% on surface preparation. The cove holds as securely as the base does — clean and strong. Skipping this stage guarantees future problems.

Surface condition assessment

First step - diagnosis. Carefully inspect the wall and ceiling area where the cornice will be mounted.

Check for strength: run your hand along the wall under the ceiling. Plaster or putty should not flake, peel, or crumble. Tap the surface with your knuckles - the sound should be dull and solid. A ringing sound indicates voids or delamination.

If the surface is loose or peeling, remove weak areas, sand down to a solid base, prime with a strengthening primer, and re-apply putty. Installing a cornice on peeling plaster will cause it to detach along with the cornice.

Check for flatness: place a long straightedge or level (2 meters) against the wall. Observe the gaps between the straightedge and the wall. Bumps and dips up to 3-5 mm are acceptable; the cornice will replicate minor irregularities or they will be filled with adhesive. Irregularities of 5-10 mm require decision-making: either level the wall or accept the gaps (they can be patched later). Irregularities over 10 mm require leveling with plaster.

Check for corner verticality: place a level vertically at the room's corners. Corners are rarely perfectly straight (90°). Deviations of 2-3° are normal and should be accounted for during cutting. Deviations of 5-10° are problematic; cutting at standard 45° will not yield a tight joint, requiring manual adjustment.

Check ceiling levelness: use a laser or water level to determine how level the ceiling is. Mark the lowest point - this will be your reference for marking. Level differences up to 10 mm across the room are acceptable. Differences over 20 mm are serious: the cornice will either be level (but with variable gaps to the ceiling) or replicate the curvature.

Our factory also produces:

View Full Product Catalog

Surface cleaning

After diagnosis - thorough cleaning.

Dust removal: vacuum the area under the ceiling using a brush attachment. Or wipe with a damp cloth and let it dry. Dust is an enemy of adhesion; adhesive won't stick to dusty surfaces.

Removal of dirt and grease stains: especially relevant in kitchens. Grease, soot, and condensation accumulate on walls and ceilings. Wipe the surface with a sponge using dish soap, rinse with clean water, and dry. Alternatively, use a spirit-based degreaser.

Removal of old finishes: if the wall has wallpaper that bubbles or doesn't adhere well, remove the area under the cornice (a strip 10-15 cm wide). Loose, peeling paint - scrape off with a putty knife and sand. Installing a cornice on wallpaper is possible but risky - the wallpaper may peel off along with the cornice.

Mold removal: if mold (black or green spots) is present in corners or under the ceiling, treat with antiseptic and let it dry. Installing a cornice over mold will allow it to continue growing underneath.

Leveling critical defects

If the mounting area has significant irregularities, cracks, or dents, correct them before installing the cornice.

Crack filling: widen the crack (using a triangular putty knife or knife), prime, and fill with gypsum or acrylic putty. After drying, sand smooth.

Filling dents: prime, fill with putty in multiple layers (if deep), and sand smooth.

Trimming bumps: if the wall has sharp bumps or protrusions, trim them with a putty knife or sand with coarse sandpaper. The cornice must lie flat against the surface.

Local leveling: if the wall area under the ceiling is heavily uneven, locally level it with a thin layer of plaster or finishing putty. Apply a 2-5 mm layer, smooth with a straightedge, let dry, and sand.

Priming

Priming - mandatory step for loose, porous, highly absorbent substrates (old plaster, porous concrete, gypsum board). It strengthens the surface, reduces absorbency, and improves adhesive bond.

Primer type: acrylic deep-penetration primer for interior use. Examples: "Ceresit CT 17", "Knauf Tiefgrund", equivalents.

Application: with a brush or roller. One coat, generously applied, without gaps. The primer should absorb well. If it absorbs immediately, apply a second coat after 30-60 minutes.

Drying: minimum 4 hours, preferably 24 hours. The primer must be fully dry before installing the cornice.

When primer is not needed: if the substrate is dense, smooth, and low-absorbent (painted walls, glossy paint) - primer is not required. Simply degrease with spirit.

Material acclimatization

While the substrate dries, take care of the cornices.

Temperature acclimatization: ifcornices for installationwere stored in a cold space (garage, balcony, warehouse in winter), bring them into the room where installation will occur, at least one day in advance. The cornice temperature must match room temperature (18-25°C). Cold polystyrene expands when heated and may deform after installation.

Unpacking: remove cornices from packaging and lay them horizontally in a dry place. Do not stand them vertically on the floor - long planks may sag under their own weight.

Inspection: check each plank for defects - chips, cracks, dents, deformations. Set aside damaged planks for short inserts in inconspicuous areas or for replacement (if defects are critical).

Marking and level control: geometry does not forgive errors

The ideal horizontal line is the basis for visual perception of the cornice. Even a deviation of 3-5 mm over a 3-meter length is noticeable to the eye. Humans subconsciously sense horizontal lines — millennia of evolution have tuned our brains to this. A curved cornice creates discomfort, even if the person cannot explain why.

Marking tools

Laser level (level): the ideal tool. Projects a strictly horizontal line onto all walls simultaneously. Can be mounted on a tripod or on the floor (if it has self-leveling functionality). Household models cost from 2000 rubles, professional ones from 8000 rubles. For one-time use, it can be rented (500-800 rubles/day).

Water level (hydrolevel): a budget alternative to the laser. A transparent tube filled with water, operating on the principle of communicating vessels. Costs 200-500 rubles, offers high accuracy but requires an assistant and takes more time.

Bubble level: a long (2-meter) construction level. Suitable for checking short sections, but inconvenient for marking the entire perimeter — error accumulates.

Tape measure: better to use a laser (more accurate and faster) or a standard steel one (minimum 5 meters).

Pencil: simple, soft (2B-4B), well-sharpened. Or a special marking pencil for construction.

Painter’s string (chalk line): for transferring lines over long distances. The string is stretched between points, pulled taut and released, leaving a straight chalk line.

Determining the base marking line

Key question: what should be used as a reference — from the ceiling or from an assumed horizontal line?

Option A: from the ceiling — if the ceiling is flat and horizontal (checked with a level, deviation no more than 5 mm). In this case, the cornice is mounted at a fixed distance from the ceiling (usually 0-10 mm). Measure this distance at several points around the perimeter; it should be the same.

Option B: from an assumed horizontal line — if the ceiling is curved, sloped, or uneven. Then mark a strictly horizontal line at a convenient height (e.g., 10 cm from the lowest point of the ceiling). The cornice is mounted along this line. The distance from the cornice to the ceiling will vary at different points, but the cornice itself will be strictly horizontal. This is the correct solution: the eye aligns with the cornice’s horizontality, not the ceiling’s curvature.

Option C: from the floor — rarely used, only if the floor is perfectly flat and horizontal, and the ceiling is very uneven. From the floor, measure the same distance along all walls (e.g., ceiling height minus 10 cm), resulting in a horizontal line.

For most cases, Option B is optimal — working from an assumed horizontal line, regardless of the ceiling’s curvature.

Marking with a laser level

Procedure:

1. Mount the laser level in the center of the room on a tripod or on the floor. The mounting height should be approximately at the level of the future cornice line (roughly 2.5–2.7 m from the floor for standard ceilings).

2. Turn on the horizontal line projection function. The laser will draw a red (or green) line on all walls.

3. Measure the distance from the laser line to the ceiling at several points along each wall. Find the lowest point of the ceiling (where the distance is minimal).

4. From the lowest point of the ceiling, measure downward a distance equal to the cornice width plus a 5–10 mm technological gap (if you do not want to mount it flush against the ceiling). For example, a 10 cm cornice with a 5 mm gap means measuring 10.5 cm downward from the lowest point.

5. Adjust the laser level’s height so the line passes at the required height. Or mark this height with a pencil and mount the laser at that mark.

6. Trace the laser line with a pencil around the entire room perimeter. This is the line for installing the bottom edge of the cornice.

Note: on textured surfaces (relief wallpaper, decorative plaster), the laser may be hard to see. Stick painter’s tape at the line’s level — the laser will be clearly visible on it. Trace the line along the tape, then remove it.

Marking with a water level

Procedure:

1. Fill the hydrolevel with water (preferably warm, as it bubbles less). Expel all air bubbles — raise the ends upward, shake until the water becomes clear without bubbles.

2. In one corner of the room, mark a point on the wall at a convenient height (e.g., 150 cm from the floor). This is the base point.

3. Attach one end of the hydrolevel to this point so that the water level (meniscus in the tube) aligns with the point.

4. Move the other end of the hydrolevel to another corner of the same wall (or to the middle if the wall is long). Hold the end against the wall, raise or lower it until the water level at both ends is equal.

5. Mark a point on the wall at the water level at the second end.

6. Repeat the procedure for all corners and intermediate points (every 2–3 meters on long walls). As a result, you will obtain several points lying on the same horizontal line.

7. Connect the points with lines using a long level, ruler, or chalk line.

8. From the resulting horizontal line, measure up or down the required distance to the curtain rod installation line (ceiling height minus rod width minus clearance).

A water level takes more time (30-40 minutes per room versus 5 minutes with a laser), but the accuracy is comparable.

Marking Corners and Joints

After marking the main horizontal line around the perimeter, mark:

Corners: At each corner, mark a vertical line with a pencil. These are where the curtain rod planks will join.

Joints on straight sections: If the wall is longer than one curtain rod plank (typically 2 or 2.5 meters), a joint is needed. Mark where it will be. Try to place joints in less noticeable locations — not in the center of the wall, but closer to the corner or behind furniture.

Openings: Doors, windows. The curtain rod is usually interrupted above openings or runs only along the top (if the opening is narrow). Mark where the rod begins and ends at the openings.

Marking Verification

Before beginning installation, verify the markings:

Levelness: Place a long level against the marked line at several points. The bubble should be exactly centered. If there are deviations, correct the markings.

Contour Closure: The line must form a closed contour around the room’s perimeter. The start and end points of the line (usually somewhere in a corner) must align at the same height. Check with a tape measure — measure the distance from the line to the ceiling at the start and end points; it should be identical.

Uniform Clearance to Ceiling: If installing with clearance, verify that it is uniform around the entire perimeter. Measure at 8–10 points. The variation should not exceed ±3 mm.

Cutting Corners in the Miter Box: Connection Accuracy

Joining curtain rods at corners — the most complex and critical installation operation. An ideal angle is when two planks meet without any gap, forming a single continuous line. A poor angle — a gap, profile misalignment, visible joint — ruins the entire appearance.

Cutting Tools

Miter Box: A device for cutting at fixed angles. A plastic miter box (300–500 rubles) is suitable for one-time use. A professional metal miter box (1500–3000 rubles) is for repeated use. A rotating miter box (from 5000 rubles) allows setting any angle — for angles other than 90°.

Handsaw: With fine teeth — for metal or plastic. Coarse teeth (for wood) tear polystyrene, leaving an uneven edge. The blade must be rigid and not bend.

Construction Knife: For narrow curtain rods (up to 5 cm wide). Sharp blade, several passes with increasing pressure. Provides a clean cut but requires a firm hand.

Electric Jigsaw: For thick and wide profiles. Blade for soft materials, medium speed, without reciprocating motion. Can cut without a miter box, setting the base blade angle to 45°.

Sandpaper: Fine grit (grit 120–180) for smoothing cut ends after cutting.

Logic of Cutting Internal Angles

Internal Angle — the most common, found in every room (angles between walls).

Principle: Two curtain rod planks meet at the angle, forming an inward angle. Each plank is cut at 45° (assuming the room’s angle is straight — 90°). The planks are joined end-to-end — resulting in a 90° angle.

Plank Orientation in the Miter Box: This is a critical moment — an error here will result in the angle being cut in the wrong place. Rule: The plank is placed in the miter box as it will stand on the wall. The part of the rod that contacts the ceiling is placed on the bottom of the miter box. The part that contacts the wall rests on the side wall of the miter box.

Cutting Direction for the Left Plank (the one going left from the corner): Cut through the slot running from left to right, top to bottom (from the far left slot to the near right slot).

Cutting Direction for the Right Plank (the one going right from the corner): Cut through the slot running from right to left, top to bottom (from the far right slot to the near left slot).

If you reverse the direction — the angle will be cut at the opposite end of the plank, not where needed.

Logic of Cutting External Angles

External Angle — protruding wall angle (column, protrusion, bay window).

Principle: Similar to internal angles, but cutting directions are opposite.

Cutting direction for the left rail: from right to left, top to bottom.

Cutting direction for the right rail: from left to right, top to bottom.

Easier to remember: for external corners, cutting directions are opposite to internal corners.

Cutting Technique

Rail Fixation: Place the rail in the miter gauge, press it tightly against the base and side wall. Hold it with your hand to prevent shifting during cutting. If the miter gauge has clamps, use them.

Saw Position: Saw blade vertical, strictly within the miter gauge slot. Do not deviate left or right — the cut will be at an incorrect angle.

Movements: Smooth, without jerks or heavy pressure. Polystyrene cuts easily — no need to press down. Use long strokes across the full blade length, not short, jerky ones.

Finishing the Cut: For the last 1–2 cm, cut more carefully, holding the piece being cut to prevent it from breaking off with the final saw stroke, leaving a chip.

Joint Inspection and Adjustment

After cutting both rails (left and right for one corner), check the joint dry — without glue.

Align the rails: Place them together end-to-end at a 90° angle, as they will stand in the room’s corner.

Assess the joint: Ideal joint — rails fit tightly, no gap, profiles perfectly match (if you run your finger along the joint, no transition is felt). Acceptable joint — micro-gap up to 0.5 mm (it will be filled with putty and become invisible after painting). Poor joint — gap 1–2 mm or more, profiles don’t match, step-like misalignment.

Adjustment: If there is a gap, determine the cause. Is the room’s angle not 90°? Was the cut not precisely at 45°? Is the rail deformed?

To fix — manually trim the end with a knife or sandpaper, removing excess material where it interferes. Re-check the fit. Repeat until you achieve a perfect joint.

Never glue without checking: Never glue rails without first checking the dry joint. Re-working after gluing is exponentially more difficult.

Cutting at Incorrect Angles

Angles in real rooms are often not 90° — they may be 87°, 93°, 95°. Cutting at standard 45° will result in a gap.

Measuring the actual angle: Use a construction protractor (angle gauge) or a protractor. Place it against the walls at the corner to determine the actual angle.

Calculating cutting angles: Divide the room’s angle in half. If the room’s angle is 94°, cut the rails at 47° (instead of 45°). If the angle is 88° — cut at 44°.

Cutting: If you have a rotating miter gauge — set the required angle and cut. If you have a standard miter gauge — you’ll need to manually adjust after standard 45° cutting by trimming the end.

Pre-cut Corner Elements

Alternative to manual trimming — pre-cut corner elements (internal and external), which manufacturers produce for some collections. They perfectly replicate the cornice profile.

Pros: No need to cut at 45°, joint is guaranteed to be perfect, installation is simpler and faster.

Cons: More expensive (corner element costs as much as 0.5–1 meter of straight rail), not available for all profiles, suitable only for 90° corners (if the angle is different — you’ll still need to adjust).

Manufacturer Hi Wood, partner of STAVROS, produces pre-cut corners for most of their collections — this significantly simplifies installation.

Choosing and Applying Glue: Chemistry of Reliable Adhesion

Glue is the only thing that holds the cornice to the wall. Whether the cornice hangs for 20 years or falls off after a month depends entirely on the correct choice and application of glue.

Types of Glue for Polystyrene

Acrylic construction adhesives — optimal choice. Water-based, solvent-free (do not degrade polystyrene), fast setting (30–60 seconds), high adhesion, can be painted or sanded.

Popular brands: "Moment Montazh Superstrong MV-50", "Titan Wild", "Quelyd Mastifix Ultra", "Eko-Gum". Available in 280–310 ml tubes for construction glue guns. Cost: 200–400 rubles per tube. Coverage: 1 tube for 15–20 meters of cornice up to 10 cm wide.

Polymer adhesives in buckets - budget option. Thick like sour cream, applied with a trowel or comb. Water-based, safe for polystyrene. Cheaper than acrylics in tubes (1 liter for 200-350 rubles vs. 200-400 rubles for 0.3 liters), but slower to set (2-3 minutes), less convenient to apply.

Brands: "Anserglob", "Krass", "Dragon". Consumption: 1 liter is enough for 25-35 meters of cornice.

Gypsum putty - an old, proven method used by professionals. Finishing gypsum putty is mixed with water to the consistency of sour cream. Cheap (5 kg bag for the whole house for 300 rubles), simultaneously bonds and fills gaps, holds firmly.

Drawbacks: messy work (putty drips), needs mixing, must work quickly (dries in 30-40 minutes), takes a long time to dry (one day). Requires skill.

Water-based liquid nails - for heavy elements. Very strong adhesion, but long setting time (5-10 minutes), requires prolonged fixation. Expensive (tube 300-500 rubles). Must be water-based, not solvent-based!

What NOT to use: silicone sealants (poor adhesion), solvent-based adhesives like "Moment", "88" (dissolve polystyrene), hot melt glue (melts polystyrene), PVA (weak adhesion, yellows).

Preparing Adhesive for Use

Acrylic adhesive in tubes: insert the tube into a caulking gun. Cut the tip of the tube at a 45° angle, with a hole diameter of 5-7 mm. Extrude a little adhesive onto a scrap piece of cardboard - this will release air, allowing the adhesive to flow evenly.

Polymer adhesive in bucket: mix the adhesive with a trowel (it may have separated during storage). Consistency - like thick sour cream. If too thick, add a little water (no more than 5%), and mix well.

Gypsum putty: pour dry mix into a bucket, add water according to package instructions (usually 0.5-0.6 liters of water per 1 kg of mix). Mix with a mixer until homogeneous. Consistency - like thick sour cream, does not flow, but is not solid. Use within 30-40 minutes, after which it will set.

Applying adhesive to cornice

How much adhesive: better to use more than less. Excess adhesive that protrudes during pressing is easy to remove. Insufficient adhesive leads to poor adhesion and detachment.

Where to apply: on the back side of the cornice - on the surfaces (legs) that will contact the wall and ceiling. NOT on the wall - on the cornice.

Application methods:

Snake (for acrylic adhesive in tubes): continuous wavy line along the entire length of each cornice leg. Adhesive roller width 5-7 mm, height 3-5 mm. Leave 5-10 mm from the edge of the leg so that when pressed, the adhesive does not squeeze out onto the front side.

Dots (for economy): large drops of adhesive every 5-10 cm. Suitable for narrow, lightweight cornices, but holds less securely than a continuous snake.

Solid line (for wide, heavy cornices): adhesive is applied as a continuous line along the entire length of the leg. Maximum contact area, maximum reliability. Can be applied with a comb.

Adhesive on ends: when joining two planks, apply adhesive to the end of the joining plank (the end that will connect to the previous one). The adhesive will fill microscopic gaps in the joint, making it monolithic.

Setting time

Some adhesives require a setting time - after application, wait 2-5 minutes until the adhesive slightly dries and becomes tacky. This improves adhesion.

Read the adhesive package instructions. Usually:

• Acrylic "Moment", "Titan" - press immediately, no setting time

• Polymer adhesives in buckets - wait 2-3 minutes

• Liquid nails - wait 5-10 minutes

• Gypsum putty - immediately, no setting time

Position adjustment

After applying adhesive, you have a limited time to adjust the cornice position:

• Fast-setting adhesives - 30-60 seconds

• Normal adhesives - 2-3 minutes

• Gypsum putty - 5-10 minutes

During this period, you can slightly move the cornice and align it with the markings. After the adhesive starts setting, it cannot be moved — adhesion will be compromised.

Pressing and Fixation: Ensuring Reliability

Applied adhesive, attached the cornice to the wall. What next? Press and hold until the adhesive sets. Sounds simple, but there are nuances.

Pressing Technique

Positioning: Before strong pressing, ensure the cornice is precisely aligned with the marking. The bottom edge of the cornice must match the applied line along its entire length. Check at several points.

Even pressing: Press the cornice evenly along its entire length. Run your palm along the entire strip, pressing each section against the wall. If you press strongly in the center and weakly at the edges, gaps will remain.

Pressing force: Moderate. Do not press too hard — polystyrene is soft and can be compressed or deformed. But also do not rub — the adhesive needs to spread, fill micro-irregularities, and ensure contact.

Removing excess adhesive: Immediately wipe away any adhesive that has squeezed out from under the cornice or from the joint using a damp cloth or putty knife. While still wet, it removes easily. Once dried, it is difficult to remove and may damage the surface.

Checking fit: After pressing, inspect the cornice. Are there gaps between the cornice and the wall or ceiling? Pay special attention to corners and joints. If the gap is small (up to 5 mm), this is normal — fill it later with putty. If the gap is large (more than 10 mm), the wall may be significantly uneven or there may be insufficient adhesive.

Holding Time

The cornice must remain pressed until the adhesive has set enough to hold the element by its own weight.

For fast-drying adhesives ("Moment", "Titan"): 30-60 seconds of active pressing with the palm, then another 1-2 minutes of holding. Total: 2-3 minutes per strip.

For regular adhesives: 2-3 minutes of active pressing, then 5-10 minutes of holding or temporarily securing with fasteners.

For gypsum putty: 3-5 minutes of active pressing, then you can release — putty sets quickly.

For liquid nails: Hold for 10-15 minutes or temporary fixation is mandatory.

Temporary Fixation

If the cornice is heavy, wide, or the adhesive sets slowly — use temporary fasteners to hold the cornice until the adhesive has fully cured.

Painter's tape: Wide (50 mm) painter's tape strips are applied from the cornice to the wall or ceiling, supporting it. Distance between strips: 30-50 cm. Tape is removed after 12-24 hours, when the adhesive has cured. Suitable for cornices of any width, universal method.

Supports: Wooden strips or boards braced against the cornice from below and to the floor. The angle of the support is adjusted to tightly press the cornice. Suitable for wide, heavy profiles. Removed after 12-24 hours.

Temporary screws: Screws are screwed into the wall (without anchors if the wall is soft, or with anchors if the wall is dense), supporting the cornice from below. Screw heads under the cornice prevent it from sliding down. After the adhesive has cured, the screws are unscrewed and the holes are filled with putty. Method for very heavy cornices or when the adhesive takes a very long time to dry (in damp rooms).

Weight from above: A heavy load (books, bricks in a bag) can be placed on the horizontal shelf of a wide cornice — pressing from above. Used rarely, more for installing cornices on ceilings (not on walls).

Features of installing the first and subsequent strips

First strip: Most important. It sets the level for all others. Install it especially carefully, repeatedly checking the level with a level. It’s better to spend 10 minutes perfectly aligning the first strip than to redo everything later.

Second and subsequent strips: Installed butt-joint to the previous one. Apply adhesive to the cornice and to the end (which will be the joint). Attach to the wall, aligning the end with the end of the previous strip. The joint must be tight, without gaps. Press the strip, paying special attention to the joint area — press the joint from both sides, achieving perfect profile alignment.

Check with a level — the new strip must be on the same horizontal plane as the previous one, without steps.

Installing Corners

Pre-cut corner elements: Installed first in the corner. Straight strips, cut at 90°, are brought up from both sides. The joint between the straight strip and the corner element is treated the same as the joint between two straight strips — adhesive on the end, tight pressing.

Manual 45° trimming: First install one strip (e.g., the left one), let the adhesive set (10-15 minutes). Then install the second (right one), carefully aligning the profiles at the joint. The corner is the most noticeable area — quality here must be maximum.

Control during installation

After installing each strip:

Checking level: Place a level against the bottom edge of the cornice. The bubble should be centered. If not — while the adhesive is still wet, adjust the position.

Check clearance to ceiling: if installing with clearance, ensure it is even. Measure with a tape measure at several points along the strip.

Check joints: carefully inspect the joint with the previous strip. Is there a gap? Do the profiles match? If anything is wrong, fix it while the adhesive is still wet.

Joint sealing: turning many into one

The cornice is installed, the adhesive has dried. But the work is not yet finished. Joints between strips, corners, gaps between the cornice and ceiling/walls — all of this must be filled with putty so that after painting, the cornice looks like a single monolithic piece.

Time to start puttying

Not earlier than one day after installation. The adhesive must be fully dry, and the cornice must hold firmly. If you apply putty earlier, the cornice may shift slightly, and the putty will crack.

Choosing putty

Acrylic putty: best choice for polystyrene. Elastic, does not crack with thermal expansion, easily sanded, compatible with water-based paints. Sold ready-to-use in buckets (no need to dilute). Brands: "Teks", "Parade", "Sniezka".

Gypsum putty: also suitable, but less elastic. May crack if the polystyrene slightly moves. However, it is cheaper than acrylic and dries faster. Suitable for stable conditions (normal temperature and humidity without sharp fluctuations).

Acrylic sealant: for filling wide gaps (over 5 mm) between cornice and ceiling/wall. Can be painted after drying. Do not confuse with silicone sealant — it cannot be painted!

Tools for puttying

Putty knives: narrow metal (5-8 cm) for applying putty, rubber (flexible) for smoothing, corner (for corners and complex profiles).

Foam: soft damp sponge for removing excess putty and forming a smooth surface.

Water container: for wetting the sponge and tools.

Cloths: for wiping tools and hands.

Filling end joints

Joints of straight cornice sections — where ends of two strips meet.

1. Take a small amount of putty onto the narrow putty knife.

2. Press the putty into the joint, run the putty knife along the joint, filling it. There should be a slight excess so that it protrudes.

3. Run the putty knife along the joint again, removing excess and smoothing the surface. Hold the putty knife at a slight angle to the surface.

4. If the joint is on a relief profile, use a thin tool (toothpick, sculpting tool, fine brush) to restore decorative lines at the joint while the putty is still wet.

5. Carefully remove excess putty from the cornice’s front surface near the joint using a damp sponge. Run the sponge along the joint to create a smooth transition from putty to polystyrene.

6. Allow to dry (4-12 hours for acrylic putty, 2-4 hours for gypsum putty).

Filling corner joints

Corners require special attention — here, two strips meet at 45° (or a pre-made corner element with two straight strips). Corners are often the most noticeable spot in the room.

The technique is the same: press putty into the joint, smooth it, restore ornament (if any), remove excess with a sponge. But work from both sides of the corner, carefully smoothing the putty at the very corner to avoid bulges.

For corners, a special corner putty knife is convenient — it has a 90° angle shape, allowing you to form a smooth surface from both sides in one pass.

Filling gaps between cornice and ceiling/wall

If there are gaps between the cornice and ceiling (or wall) due to uneven base — fill them with putty or sealant.

Narrow gaps (up to 5 mm): acrylic putty. Press into the gap with a narrow putty knife, smooth it. After drying, sand it.

Wide gaps (5-10 mm): acrylic sealant. Extrude from the tube into the gap, smooth with a wet finger or putty knife, forming a smooth transition. After drying, you can paint.

Very wide gaps (more than 10 mm): first fill the gap with polystyrene pieces (trimmings from crown molding) or construction foam (carefully, sparingly, to avoid overfilling). Let it dry. Then skim coat or seal the top.

Multi-layer spackling

Often, one layer of spackling is insufficient. After drying and sanding, micro-unevenness, roughness, or pores may remain.

Apply a second layer of spackling — thin, only on problem areas. Dry and sand. If necessary, apply a third layer. Goal: a perfectly smooth joint surface, indistinguishable from the original polystyrene.

Sanding, priming, painting: final finish

Spackling has dried. Now you need to sand, prime, and paint — transforming white polystyrene into an interior element.

Sanding spackled joints

Tools: fine-grit sandpaper (grit 120-180), sanding block (wood block with sandpaper) or simply sandpaper wrapped around a wooden block.

Technique: sand the spackled area along the joint with light, even strokes. Goal: smooth the transition from spackling to polystyrene, remove micro-unevenness. Do not press hard — polystyrene is soft and can be sanded through.

Check: run your finger along the joint. The transition from spackling to polystyrene should be imperceptible to the touch. If you feel a step or bump, continue sanding.

Dust removal: after sanding, fine dust is formed. Brush it off with a soft brush or blow it away. Wipe the crown molding with a slightly damp cloth, then let it dry.

Priming before painting

Priming is not mandatory, but recommended for an ideal result.

Why prime?

• Equalizes surface absorption (polystyrene and spackling absorb paint differently; primer equalizes this)

• Improves paint adhesion

• Reduces paint consumption (primer is cheaper than paint)

• Fills micro-pores, creates a smooth base

Type of primer: water-based acrylic primer. Universal or special "for plastic". White (if painting white) or tinted to match future paint color.

Application: soft brush 5-7 cm wide or small velvet roller. Thin layer, no drips. Primer should be absorbed. One layer is sufficient.

Drying: 2-4 hours at room temperature. After drying, lightly sand with fine grit paper (grit 180-220) — this improves paint adhesion.

When primer is not needed: if painting with quality white paint (latex or acrylic), primer can be skipped. The paint will adhere well anyway.

Painting the crown molding

Painting timing: either before installation (planks painted on the floor, then mounted; joints touched up after installation), or after installation and spackling (paint everything at once). The second option is preferred — no risk of damaging paint during installation.

Type of paint: only water-based paints — emulsion, acrylic, latex. No solvent-based paints (oil, alkyd, nitro enamel) — they will dissolve polystyrene.

Paint selection:

• Emulsion — cheapest, matte, suitable for dry rooms

• Acrylic — better, satin or semi-gloss, washable, UV-resistant

• Latex — premium, silky, moisture-resistant, for kitchens and bathrooms

Color: white — classic. Colored — tinted at the store to the desired shade or add tint at home. Add tint gradually, in small portions, mix well, check the shade on a cardboard sample.

Tools: a soft synthetic brush 5-7 cm for the entire surface, a brush 2-3 cm for hard-to-reach areas and relief. Or a small velvet roller 10 cm for smooth profiles.

Paint preparation: mix, dilute with water if necessary (no more than 5-10%). Consistency — like liquid sour cream.

Protect walls and ceiling: apply painter’s tape to the wall under the cornice and to the ceiling above the cornice (if the cornice is not flush). This protects against accidental brush strokes. Apply tape carefully, flush against the cornice.

Application of the first layer:

1. Dip the brush into paint up to one-third of the bristle length, lightly squeeze out excess paint at the edge of the can.

2. Apply paint along the length of the cornice with light strokes. Do not press the brush too hard, avoid thick layers — better two thin layers than one thick one.

3. On decorative profiles, carefully paint recesses. Paint should not accumulate in recesses — wipe off excess with a dry brush or cloth.

4. Work in sections of 1-2 meters, do not let paint dry at section boundaries — otherwise, streaks will remain.

Drying: water-emulsion and acrylic paints dry to the touch in 2-4 hours, fully in 12-24 hours. Do not apply the second layer until the first is completely dry.

Second layer: applied the same way as the first. After the second layer, the coating should be even, dense, and free of gaps. If the substrate is visible, a third layer is needed.

Removing painter’s tape: remove tape while paint is still slightly damp (after 1-2 hours of applying the final layer). If paint is fully dry, tape may peel off with paint, leaving an uneven edge.

Touch-up at joints: after full drying, inspect joints. If any spackle marks or color unevenness are visible, touch up with a fine brush in spots.

Decorative Painting Techniques

If you want not just a painted cornice, but with effects:

Patina effect: first layer — light paint (white, cream). After drying — second layer with dark paint (brown, gray, black). While still wet, wipe off dark paint from raised parts of the relief with a dry cloth. Dark paint remains in recesses — vintage effect.

Dry brush (gilding): base layer — main color. After drying, take a minimum amount of gold/silver paint with a stiff brush. Wipe brush almost dry. With a semi-dry brush, lightly stroke raised parts of the relief. Result — light gilding.

Two-color painting: main part of cornice — one color, ornament — another. First, paint everything in the base color. After drying, cover the ornament with painter’s tape, and the rest with paper. Paint the ornament in the second color. Remove tape.

Installing cornices with LED strip: light and decoration in one

If using a cornice with a recess for lighting, installation becomes more complex — you must not only attach the cornice but also integrate the lighting equipment.

Work sequence

1. Base preparation, marking (as usual)

2. Cornice installation (as usual)

3. Drying time (one day until adhesive fully sets)

4. Installing LED strip in recess

5. Electrical connection (power supply, control)

6. Testing the lighting

7. Joint sealing, sanding, painting (as usual, but carefully to avoid damaging the strip)

Preparing the cornice recess

The inner surface of the recess, where the strip will be glued, must be clean, dry, and degreased.

Wipe with alcohol or degreaser. If the surface is dark, you can cover it with aluminum foil or paint it white — this increases light output by 15-25%.

Installing LED strip

Measuring and cutting the strip: measure the length of each niche section (along each wall). Cut the strip only at designated marks (usually every 5-10 cm, marked with scissors or a line). Do not cut elsewhere — this may damage the conductive trace.

Adhesive application: remove the protective film from the adhesive layer on the back of the strip. Apply the strip into the niche, pressing it firmly. The strip must lie flat, without bends. Diodes must face the correct direction (up toward the ceiling or down toward the wall — depending on the type of lighting).

Additional fixation: the adhesive layer of the strip is not very reliable. Secure the strip additionally with special clips (sold in electrical stores) every 30-50 cm. Alternatively, use an aluminum profile for LED strips — the strip is inserted into the profile, which simultaneously secures and dissipates heat.

Connecting segments at corners: at internal and external corners, the strip is either trimmed and connected with special connectors (fast, but less reliable), or soldered (reliable, but requires skill). Connectors are plastic clips with contacts into which the ends of the strip are inserted. Soldering involves soldering wires to the strip’s contact pads using tin solder.

Avoid sharp bends: LED strip does not like sharp bends (radius less than 5 cm). At corners, make a smooth turn or use connectors.

Wiring and connection

Wires from the strip to the power supply: use multi-strand copper wire with cross-section 0.75–1.5 mm² (depending on power). Run wires concealed — behind the cornice, in grooves, behind drywall structures.

Polarity compliance: LED strip is polarized. Positive to positive, negative to negative. Usually, the positive wire is red, the negative is black or blue. If connected incorrectly, the strip will not light up (but won’t burn out — it simply won’t work).

Connecting to the power supply: the unit has input terminals (220V, connect the mains wire) and output terminals (12V or 24V, connect the strip wires). Observe the markings and tighten the terminals securely.

Installing a switch/dimmer: between the power supply and the strip (on the low-voltage side) or between the mains and the power supply (on the 220V side). The first option is safer and technically more correct.

Power supply placement: in an accessible location (for maintenance), with ventilation (the unit heats up). Behind the cornice, in a niche, in a technical box. Do not seal it completely.

Testing the lighting

Before sealing joints and painting, test the lighting.

Turn on: everything should light up evenly, without gaps, dark spots, or flickering.

Check uniformity: is the light evenly distributed across the ceiling/wall? Are there no stripes or spots?

Check strip concealment: from any point in the room, individual LEDs should not be visible. If visible, the cornice lip is too low or the strip is positioned incorrectly.

Check control: does the switch/dimmer work correctly? Is brightness smoothly adjustable (if using a dimmer)?

Identified issues: fix before painting. After painting, access to the strip will be difficult.

Safety techniques during installation

Installing cornices may seem safe, but injuries are possible. Follow safety rules.

Working at height

Ladder or scaffolding: use a stable, sturdy ladder with wide steps. Height should allow you to work at chest level, not stretching upward. Check stability before climbing. On slippery floors, place rubber mats under the ladder feet.

Forbidden: standing on stools, tables, or unstable furniture. Reaching far sideways while standing on a ladder (risk of losing balance). Working on a ladder with two people.

Safety harness: when working at high heights (over 2.5 m), a safety harness is recommended — a helper who holds the ladder steady.

Working with tools

Sharp tools: hacksaw, knife, scissors. Cut away from yourself, not toward yourself. Do not place fingers under the blade. Store in cases when not in use.

Electric tools: electric jigsaw. Disconnect from power when changing blades. Do not touch the moving blade. Work with safety goggles (dust and shavings fly).

Adhesive gun: when pressing the trigger, adhesive is expelled under pressure. Do not point the nozzle at yourself or others. When expelling adhesive, hold the gun with both hands.

Working with chemicals

Adhesives: work in a well-ventilated room. Most modern water-based adhesives are safe, but still avoid prolonged inhalation of fumes. If adhesive contacts skin, wash with water. If it gets into eyes — thoroughly rinse and consult a doctor.

Paints: water-based paints are safe, but ventilate the room. Work with gloves (paint dries skin). Protect eyes — if paint gets in, rinse with water.

Solvents: used only for degreasing surfaces (alcohol). Flammable — do not smoke, do not work near open flames. Fumes are toxic — work in a well-ventilated room.

Electrical safety when installing LED lighting

De-energize: perform all electrical work (connecting the power supply to 220V network) with the circuit breaker turned off in the panel. Check for absence of voltage using a voltage tester.

Insulation: carefully insulate all wire connections with electrical tape or heat-shrink tubing. There should be no exposed wires.

Qualification: if you are unsure of your electrical knowledge, entrust connecting to 220V network to a professional electrician. The low-voltage part (12V/24V) you can do yourself — it is safe.

Do not turn on until completed: do not turn on the lighting until the installation and insulation of all connections are fully completed.

Dust Protection

During sanding, fine dust (from putty and polystyrene) is produced. It irritates the respiratory tract.

Respirator: disposable mask or reusable respirator. Protects the lungs.

Ventilation: sand with windows open. After work, vacuum the room.

Eye protection: wear safety goggles during intensive sanding (especially overhead).

Final acceptance: quality control of work

Installation is complete, paint is dry. Before considering the work finished, conduct a detailed acceptance check — evaluate quality against all parameters.

Visual inspection checklist

Levelness of the line:

• Step back to the opposite wall and view the cornice from afar

• The line must be strictly horizontal, without waves, misalignments, or unevenness

• Place a long level at several points — the bubble should be centered

• Permissible deviation: ±2 mm per 2 meters of length

Joint quality:

• Inspect each joint up close

• Joints must be tight, with no visible gaps

• Profiles at the joint match perfectly, without steps or misalignment

• Run your finger along the joint — the transition should not be felt

• On decorative profiles, the ornament is continuous across the joint

Angles:

• Most noticeable and critical areas

• Planks meet at an exact angle, with no gaps

• Relief at angles is continuous

• Putty (if any) is not noticeable after painting

Fit to wall and ceiling:

• The cornice fits tightly against the wall along its entire length

• No gaps, sagging, or gaps exceeding 3-5 mm

• The gap between the cornice and the ceiling (if any) is even

Paint quality:

• Coverage is even, without gaps, voids, or streaks

• Color is uniform along the entire length

• No drips, runs, or stains

• The relief is fully painted; paint does not accumulate in recesses

• Edges with walls and ceiling are sharp (if painter's tape was used)

No damage:

• No dents, scratches, or chips on the front surface

• No glue, dirt, or fingerprints

• Surface is clean and neat

LED lighting check (if applicable)

Turn on: lighting turns on immediately, without delay or flickering

Lighting uniformity:

• Light is evenly distributed along the entire length

• No dark spots, dips, or overexposed areas

• No visible stripes from individual LEDs

Light source concealment:

• Stand at different points in the room

• The LED strip should not be visible directly

• Only reflected light on the ceiling/wall is visible

Control:

• Switch/dimmer works properly

• Brightness adjusts smoothly (if a dimmer is installed)

• Color matches the ordered one (warm/neutral/cold white or RGB)

No extraneous sounds:

• Lighting operates silently

• No hum from power supply, buzzing, or crackling

Checking mounting reliability

Light touch: gently press the cornice at several points. It should be firmly seated, not sagging, and not pulling away from the wall.

Tapping: gently tap the knuckles of your fingers along the entire length of the cornice. The sound should be dull and uniform. A resonant sound in certain spots indicates voids or poor adhesion.

Checking corners: corners are the most heavily loaded areas. Check them especially carefully. Gently press on the corner — it must hold absolutely rigidly.

Inspection at different times of day

Inspect the cornice under different lighting:

Daylight: under natural light from windows. How does the cornice look? Is the shadow line pronounced? Are the joints invisible?

Evening artificial lighting: with chandeliers and wall sconces on. Defects may become apparent under different lighting angles, which are not noticeable during the day.

Cornice lighting (if present): evaluate the effect with LED lighting turned on. Does it meet expectations? Is the brightness sufficient? Is the light color correct?

Photo documentation of the result

Photograph the completed work from different angles:

• General room shots with the cornice

• Close-up shots of corners and joints

• Photos with lighting (if present) at different brightness levels

Photos will be useful for a portfolio (if done for a client), for "before/after" comparison, and for analyzing mistakes (if there are any flaws — you’ll take them into account for the future).

Correction of identified defects

If defects are found during acceptance — correct them before handing over the object (if working on commission) or until you begin living in the renovated interior.

Visible joints: additionally spackle, sand, and repaint.

Unpainted areas: repaint missed areas with a fine brush.

Minor paint defects: sand lightly with fine sandpaper, then repaint.

Detached sections: reattach, secure, and wait for the adhesive to set.

Non-functioning lighting: check connections, polarity, and integrity of the strip. Repair it.

Conclusion

Installing decorative polystyrene cornices — a step-by-step, regulated process. Each stage is important, and every operation affects the final result. Skip preparing the base — the cornice will detach. Carelessly cut corners — joints will have gaps. Poorly apply adhesive — gaps and sagging will appear. Fail to spackle seams — they will be visible after painting. The technology does not forgive negligence.

But if you follow the procedure step by step — from surface preparation to final acceptance — the result is guaranteed. Even a person without experience in finishing work, but attentive and careful, will performcornice installationat a professional level. The key is not to rush, control each stage, correct flaws immediately, rather than leaving them "for later".

High-quality materials simplify the work. Cornices with precise geometry fit together without gaps. Dense polystyrene cuts cleanly without crumbling. Clear relief looks like real molding after painting. The company STAVROS offers a wide range of cornices from a reliable partner-manufacturer Hi Wood (HiWood) — materials tested in thousands of projects, with stable quality and thoughtfully designed geometry.

STAVROS professional consultants will not only help you select the right profile but also provide detailed installation recommendations for a specific collection. They will explain the characteristics of cutting different reliefs, recommend the optimal adhesive and paint, and advise how to avoid typical mistakes. This is especially valuable for those installing cornices for the first time — professional support at every stage, from purchase to final acceptance.

By following this step-by-step procedure, using high-quality materials from reputable manufacturers, and not skipping any stage of the technology, you will create a decorative ceiling trim that will delight you with its flawless appearance for many years. A perfectly straight horizontal line, invisible joints, perfect angles, even paint application, and striking lighting — all of this is achievable with your own hands. Successful installation!