How to Plaster a Wall: UK Guide

Overview of the plastering process

Plastering a wall in the UK typically follows a two-coat system. The first coat (undercoat) provides a level base and builds up thickness. The second coat (finish coat or skim) creates the smooth final surface ready for decoration.

On brick, block, or concrete you apply an undercoat (bonding, browning, or hardwall depending on suction) at 8-11mm thickness. Once firm, you key it, then apply multi-finish at 2-3mm and trowel to a polished surface.

On plasterboard you usually apply just a skim coat of board finish or multi-finish at 2-3mm. No undercoat is needed because the board provides the flat base. This guide covers both scenarios.

Tools and materials needed

Essential tools

• Plastering trowel: 11-inch or 13-inch laying-on trowel. Carbon steel, flexible blade.
• Hawk: Flat square board with handle underneath. Holds plaster while you work.
• Mixing paddle: Spiral mixer attachment for a power drill. Mixes plaster quickly and lump-free.
• Bucket: Clean plastic bucket, 15-20 litre capacity for mixing.
• Darby or feather edge: Long straight edge (1.5-2m) for levelling undercoat.
• Scratcher or comb: For keying the undercoat.
• Brushes and sponges: For dampening surfaces and cleaning tools.

Materials

• Undercoat plaster: Bonding (low-suction backgrounds), browning (moderate-suction), or hardwall (high-suction).
• Finishing plaster: Multi-finish (most backgrounds) or board finish (plasterboard only).
• PVA primer: Diluted adhesive for sealing dusty surfaces or painted walls.
• Water: Clean cold water for mixing.

Step 1: Prepare the surface

Surface preparation determines whether the plaster sticks. Poor prep causes delamination and cracking.

Brick, block, or concrete

Brush off dust, loose mortar, and debris. Rake out crumbling joints to at least 10mm depth and fill with sand-cement mortar. Let repairs cure for 24 hours. On very smooth concrete, score the surface with a bolster chisel to provide a key. If the wall is dusty or friable, apply diluted PVA (1:5 with water) and let it dry, then apply a second coat at 1:3 and plaster onto the wet PVA.

Plasterboard

Ensure all joints are taped and filled. Screw heads should be just below the surface. Brush off dust. Some plasterers apply diluted PVA (1:5) to reduce suction and extend working time. Others skim directly onto the board with no primer. Both methods work.

Painted surfaces

Remove any loose or flaking paint. Key the surface with coarse sandpaper or a scarifier. Apply two coats of diluted PVA as above. Use bonding plaster as the undercoat because painted surfaces have low suction.

Step 2: Apply the undercoat (two-coat plastering)

Skip this step if you are skim-coating plasterboard. For brick, block, or concrete, the undercoat is essential.

Mix the undercoat plaster

Use bonding, browning, or hardwall depending on background suction. Add plaster powder to clean cold water in the bucket. Typical ratio is 0.5 litres of water per kg of plaster (for a 25kg bag, use 12-13 litres). Mix with the paddle mixer until smooth and creamy, like thick custard. Let it stand for 2 minutes, then give it a quick remix.

Load the hawk and apply to the wall

Scoop plaster onto the hawk with the trowel. Hold the hawk in your left hand (if right-handed) close to the wall. Slide the trowel across the hawk to pick up a thin layer of plaster, then press it onto the wall with an upward sweeping motion. Build up the coat gradually to 8-11mm thickness.

Level and flatten

Use the darby or feather edge to level the plaster. Work from bottom to top, using a sawing motion to remove high spots and fill hollows. Check with a spirit level and straight edge. The surface does not need to be smooth (that is the job of the finish coat) but it must be flat and even in thickness.

Key the surface

After 40-60 minutes the plaster will firm up. Test by pressing with your thumb. When it resists but still has slight give, use the scratcher or edge of the trowel to score horizontal lines across the surface at 50mm intervals. This creates a key for the finishing coat. Do not key too early (the plaster will drag) or too late (it will be too hard).

Step 3: Apply the finishing coat

The finishing coat (multi-finish or board finish) goes on top of the undercoat or directly onto plasterboard. This is the coat that gets trowelled to a smooth polished surface.

Timing

Wait 2-3 hours after applying the undercoat. It should be firm but still slightly damp. If it has dried out completely, dampen it lightly with a brush. Do not soak it. On plasterboard, you can start immediately after priming (if using PVA).

Mix the finishing plaster

Add multi-finish powder to clean cold water. Use slightly more water than for undercoat: around 0.6 litres per kg (for a 25kg bag, 14-15 litres). Mix until smooth and creamy, thinner than undercoat but not runny. Consistency like thick yogurt.

First trowel pass

Apply the finishing plaster at 2-3mm thickness using the same hawk and trowel technique. Work in vertical strokes, building a thin even layer. Cover the whole area quickly. Do not worry about trowel marks at this stage. Just get it on evenly.

Second trowel pass

Wait 10-15 minutes for the plaster to start firming up. Lightly spray or flick water onto the surface with a brush. Trowel again with firm pressure, holding the trowel at a shallow angle. This flattens trowel marks and starts to polish the surface. Work in overlapping arcs.

Final trowelling and polishing

After another 10-15 minutes the surface will be almost set. Spray it lightly again and trowel with very firm pressure, almost burnishing the plaster. The surface should become smooth and glass-like. Any remaining trowel marks can be removed with a final very light pass using the trowel held almost flat. Do not over-trowel or you will bring water to the surface and cause ripples.

Single coat vs two-coat plastering

Two-coat system

Used on brick, block, concrete, or any rough background. Undercoat (8-11mm) provides the level base. Finish coat (2-3mm) provides the smooth surface. Total thickness 10-14mm. Takes 4-5 hours start to finish. Stronger and more durable. Industry standard for solid walls.

Single coat (skim only)

Used on plasterboard or over existing smooth plaster. No undercoat, just a thin skim of finishing plaster at 2-3mm. Takes 2-3 hours. Easier for DIYers because you do not need to level or key an undercoat. Only works if the background is already flat.

Which to use?

New plasterboard: single coat skim. Brick, block, or concrete walls: two-coat system. Replastering over old lime or gypsum plaster: depends on condition. If the old plaster is flat and sound, skim it. If it is uneven or blown, hack it off and two-coat from scratch.

Common mistakes and how to avoid them

Mix too wet

Plaster runs or sags on the wall. Caused by adding too much water. Stick to the recommended ratio. Better to mix slightly stiff and adjust than to add too much water at the start. You cannot fix plaster that is too wet. Throw it away and remix.

Wrong timing between coats

Applying the finish coat too early causes it to slide or mix with the undercoat. Too late and it does not bond properly. Wait until the undercoat is firm to thumb pressure but still damp. Usually 2-3 hours, but test by feel rather than relying on the clock.

Trowelling too early or too late

Trowelling the finish coat when it is still wet pushes it around and creates ridges. Trowelling when it is too dry tears the surface. Trowel in stages as the plaster firms up: first pass to flatten, second pass to polish, final pass to burnish. Each pass happens when the plaster has stiffened slightly more.

Dirty tools or water

Old plaster stuck to the trowel or hawk falls into the new mix and creates lumps. Dirty water accelerates setting. Clean all tools thoroughly between mixes. Use fresh water for each batch.

Not priming dusty or low-suction surfaces

Plaster delaminates or dries too fast. Always PVA dusty or painted surfaces. On very dense backgrounds (polished concrete, tiles), score the surface mechanically or the plaster will not grip.

Drying and curing times

Plaster sets quickly but dries slowly. Setting (going hard) is a chemical reaction that happens in 1.5-2 hours. Drying (losing moisture) takes days or weeks.

• Set time: 1.5-2 hours. Plaster is hard but still wet inside.
• Surface dry: 24-48 hours. Colour changes from dark to light. Safe to touch and decorate in low-humidity areas.
• Fully dry: 4-6 weeks depending on thickness and ventilation. All moisture has evaporated. Essential before tiling or hanging wallpaper.

Ventilate well but avoid forced drying with heaters. Let the plaster dry naturally. Paint too early and you seal moisture into the wall, causing damp patches and peeling paint. Use a moisture meter if in doubt.

DIY vs hiring a professional plasterer

When to DIY

Skimming a small plasterboard area (ceiling patch, single wall in a bedroom) is achievable for a competent DIYer. The board provides the flat surface so you only need to master trowelling. Expect your first attempts to show some imperfections. Practice on scrap board first.

When to hire a pro

Two-coat plastering on brick or block requires skill and experience to achieve a flat base coat. Getting the undercoat level is harder than finishing. Whole-room or whole-house jobs are faster and better done by professionals. A plasterer will complete a room in a day to a high standard. A DIYer might take a week and still have an uneven finish.

Cost of hiring a plasterer

Expect to pay £150-300 per day (region and experience dependent). Skim coat for a small bedroom (walls only): £250-400. Full two-coat plastering for a 4m x 4m room: £500-800. Prices include materials and labour. Get three quotes and check previous work.

Coverage and material quantities

Material quantities for a typical 4m x 4m room (walls only, 2.4m high, total area around 35m²):

Two-coat system

• Undercoat (11mm): 14 bags bonding or browning
• Finish coat (3mm): 7 bags multi-finish
• PVA primer: 2 litres
• Total material cost: £150-200

Single coat skim on plasterboard

• Finish coat (3mm): 7 bags multi-finish
• PVA primer (optional): 1 litre
• Total material cost: £50-70

Coverage per bag: bonding or browning covers 2.5m² at 11mm. Multi-finish covers 5m² at 3mm. Always buy 10% extra for wastage.

Related guides

• Bonding Plaster: UK Guide
• Finishing Plaster Explained
• Plasterboard: Complete Guide
• Plasterboard Jointing Techniques