How much multi-finish do I need per m²?

Multi-finish plaster (such as British Gypsum Thistle Multi-Finish) covers approximately 10 m² per 25kg bag when applied at the standard 2mm skim coat thickness on plasterboard (per British Gypsum datasheet). Coverage on old plaster is less (~8 m²/bag) due to the more uneven and absorbent surface.

What thickness should a skim coat be?

A standard skim coat in the UK is applied at 2-3mm thickness. This is the finishing coat applied over plasterboard, backing coat, or existing plaster. Two thin passes are better than one thick coat — the first pass fills imperfections and the second gives the smooth finish.

Can I skim over old plaster?

Yes, you can skim over old plaster if it is still firmly bonded to the wall. First, apply a coat of PVA (diluted 1:4 with water) as a bonding agent and let it go tacky. Then apply your skim coat. If the old plaster is blown (hollow-sounding when tapped), it needs to be hacked off and re-done.

What is the difference between skim coat and plastering?

Skim coating (or skimming) refers specifically to the thin 2-3mm finishing coat of multi-finish plaster. Full plastering involves applying a thicker backing coat (8-11mm of bonding or hardwall) first, then the skim coat on top. Skim coating alone is done over plasterboard or sound existing surfaces. Use our Plaster Coverage Calculator for bonding and undercoat quantities.

How long does skim coat take to dry?

A 2-3mm skim coat typically sets within 1.5-2 hours and can be trowelled to a smooth finish. However, it takes 2-3 days to fully dry before painting. The plaster will turn from dark to light pink/white as it dries. Never paint or wallpaper over damp plaster. Use our Plaster Drying Time Estimator for a more precise estimate based on your room conditions.

Skim Coat Calculator — How Much Multi-Finish Do I Need?

Calculate how many bags of multi-finish plaster you need for skim coating walls and ceilings. Based on UK 25kg bag sizes.

Input Method

Choose how to enter your measurements

Total Area to Skim

Enter the total surface area to be skim coated

Surface Type

Coverage varies based on the surface being skimmed

Include wastage allowance

Adds 10% extra for mixing and application losses

Wastage Percentage

10% for experienced plasterers, 15-20% for DIY

Price Per Bag (optional)

Enter your supplier price for a cost estimate

How We Calculate This

Our skim coat calculator divides your total wall or ceiling area by the coverage rate of multi-finish plaster based on your surface type. Coverage rates are based on British Gypsum Thistle Multi-Finish guidelines at the standard 2mm application thickness.

The formula

Bags needed = (Total area ÷ Coverage per bag) × Wastage factor

For example, skimming 20 m² of plasterboard at 10 m²/bag: 20 ÷ 10 = 2.0 bags → round up to 2 bags, plus 10% wastage margin = 3 bags of 25kg multi-finish.

Coverage rates by surface

Plasterboard: 10 m² per 25kg bag at 2mm — smooth, consistent surface absorbs less
Backing coat: 10 m² per 25kg bag at 2mm — standard rate over bonding or hardwall
Old plaster / patched walls: 8 m² per 25kg bag at 2mm — uneven surfaces require more plaster

These rates assume a 2mm skim coat thickness as recommended by British Gypsum. The 10% wastage factor covers mixing losses, dropped plaster, and filling minor imperfections. If you are new to skimming, consider 15-20% wastage.