Finishing Plaster: UK Guide

What is finishing plaster?

Finishing plaster (also called skim plaster or topcoat plaster) is a fine gypsum-based plaster designed to be applied as the final layer over an undercoat or directly onto plasterboard. It is trowelled to a smooth, polished finish ready for decoration with paint or wallpaper.

Unlike undercoat plasters (bonding, browning, hardwall) which are coarse and build thickness, finishing plaster has a very fine particle size and is applied thinly (2-3mm). It sets quickly (1.5-2 hours) and can be polished to a glass-like surface with repeated trowelling.

The most common UK finishing plasters are British Gypsum Thistle Multi-Finish and Thistle Board Finish. Other brands include Knauf Multi-Finish and Siniat Sirapite Finish.

Multi-finish vs board finish

Multi-finish (most versatile)

Works on a wide range of backgrounds: plasterboard, bonding/browning undercoat, old gypsum or lime plaster, and painted surfaces (if keyed). Sets in 1.5-2 hours. Most widely stocked finishing plaster. Use when you are unsure which finish to choose. Costs £7-10 per 25kg bag.

Board finish (plasterboard only)

Formulated specifically for direct application onto plasterboard. Sets slightly slower than multi-finish (2-2.5 hours), giving more working time on large ceiling areas. Only use on plasterboard. Do not use on undercoat plaster or other backgrounds. Costs £7-10 per 25kg bag.

Which to use?

Multi-finish for most jobs (walls with undercoat, plasterboard, mixed backgrounds). Board finish only for large plasterboard areas where the extra working time is helpful (ceilings, open-plan rooms). In practice, many plasterers use multi-finish for everything because it is more versatile and performance is similar.

Application thickness

Finishing plaster should be applied at 2-3mm thickness. This is very thin compared to undercoat (8-11mm) or screed (50mm+). Apply it in one pass, building an even layer across the surface.

Why 2-3mm?

• Thinner than 2mm: Prone to cracking, uneven coverage, and showing the background through. Not enough material to trowel to a polished finish.
• Thicker than 3mm: Takes too long to set. Heavy and prone to slumping or sagging on walls. Difficult to polish because the surface stays soft. Wastes material and time.

Experienced plasterers apply at exactly 2mm for speed and economy. DIYers should aim for 2-3mm and accept slight variation. Measure thickness by feel and by checking the level against the background.

Mixing finishing plaster

Use a clean bucket and clean cold water. Add plaster powder to water (never water to powder or it will lump).

Water-to-plaster ratio

Approximately 0.6 litres of water per kg of plaster. For a 25kg bag, use 14-15 litres of water. The mix should be smooth and creamy, like thick yogurt. Slightly thinner than undercoat plaster but not runny.

Mixing method

Pour water into the bucket first. Sprinkle the powder evenly into the water. Let it slake (absorb water) for 1-2 minutes. Then mix by hand or with a paddle mixer until smooth and lump-free. Do not over-mix or the plaster will start setting in the bucket.

Working time

Finishing plaster has a working time of 30-40 minutes from mixing to when it becomes too stiff to spread. In hot weather or with old plaster, working time reduces. Mix only as much as you can use in 30 minutes. Never add water to plaster that has started setting. Throw it away and mix fresh.

Trowelling technique

Finishing plaster is applied and smoothed in stages as it sets. The key to a good finish is trowelling at the right time.

First pass: apply the plaster

Load your hawk with mixed plaster. Use a laying-on trowel (11-13 inch) to spread plaster onto the wall or ceiling in upward sweeping strokes. Build an even layer 2-3mm thick. Cover the whole area quickly. Do not worry about trowel marks at this stage. Just get it on evenly.

Second pass: flatten (after 10-15 minutes)

Wait for the plaster to start firming up. Test by pressing with your finger. When it resists but still has slight give, it is ready. Spray or flick a little water onto the surface, then trowel with firm pressure at a shallow angle. This flattens trowel marks and starts to compress the surface. Work in overlapping arcs.

Third pass: polish (after another 10-15 minutes)

The plaster is almost set. Spray it lightly again and trowel with very firm pressure, almost burnishing. The surface should become smooth and shiny. Any remaining marks can be removed with a final light pass. Hold the trowel almost flat and skim over the surface with minimal pressure.

Common mistake: trowelling too early or too late

Trowelling when the plaster is too wet pushes it around and creates ridges. Trowelling when it is too dry tears the surface and creates scratches. Each trowel pass must wait until the plaster has stiffened slightly from the previous pass. This is learned by feel and takes practice.

Coverage and material quantities

A 25kg bag of finishing plaster covers:

• 12-13m² at 2mm thickness
• 10-11m² at 2.5mm thickness
• 8-9m² at 3mm thickness

Example calculation

Room with 30m² of wall area. At 2.5mm thickness: 30 ÷ 10 = 3 bags. Add 10% for wastage = 4 bags total. Cost: 4 x £8 = £32.

For a whole house (120m² walls and ceilings), you need around 12-15 bags of multi-finish (£90-120 in materials).

Drying and curing

Finishing plaster sets (goes hard) in 1.5-2 hours. Full drying (losing all moisture) takes 1-2 weeks depending on thickness, ventilation, and ambient conditions.

Setting vs drying

• Setting: Chemical reaction. Plaster goes from soft to hard. Happens in 1.5-2 hours. Irreversible.
• Drying: Physical evaporation. Plaster loses moisture and changes colour from dark to light pink. Takes 1-2 weeks.

When can I decorate?

Wait until the plaster is fully dry. The colour will be uniform light pink with no dark patches. If you paint too early, moisture gets trapped behind the paint and causes damp patches, peeling, or mould. Use a moisture meter if in doubt. Maximum 1-2% moisture content for emulsion paint, lower for wallpaper.

Mist coat

Before applying full-strength paint, apply a mist coat (diluted emulsion, 10-20% water). This seals the plaster and provides a good base for the topcoat. Essential on new plaster. Skipping the mist coat causes patchy coverage and poor adhesion.

Common problems and solutions

Cracking

Fine cracks appear as the plaster dries. Caused by applying too thick, mixing too wet, or rapid drying (direct heat or draughts). Stick to 2-3mm thickness and let the plaster dry naturally. Ventilate but avoid fan heaters.

Ripples or trowel marks

Visible marks in the finished surface. Caused by insufficient trowelling or bringing water to the surface by over-trowelling. Trowel in stages as the plaster firms. Each pass should be firmer than the last. Stop when the surface is smooth and shiny.

Patchy drying

Some areas dry faster than others, leaving dark and light patches. Usually caused by uneven application thickness or variable suction in the background. Ensure even thickness and prime high-suction backgrounds before skimming.

Poor adhesion

Plaster delaminates or sounds hollow. Caused by dusty background, insufficient PVA priming, or applying to a completely dry undercoat. Always dampen or PVA the background before applying finishing plaster.

Cost and budget

Material costs for finishing plaster:

• Multi-finish or board finish: £7-10 per 25kg bag
• PVA primer (if needed): £5-8 per litre

Labour costs (hiring a plasterer)

Plasterers charge £150-300 per day depending on region and experience. A skilled plasterer can skim 20-40m² per day. Typical room (30m²): £200-350 including materials and labour. Whole house (120m²): £1,200-2,000.

Related guides

• How to Plaster a Wall: UK Guide
• Bonding Plaster: UK Guide
• Plasterboard: Complete Guide
• Plasterboard Jointing Techniques