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Project guide

How much plaster do I need to skim a room?

A finish skim is about 2mm thick. British Gypsum says a 25kg bag of Multi-Finish covers about 10 m² at that thickness.

Updated Based on manufacturer guidance, sources below

The short answer

Bags of Multi-Finish = area to skim x 1.1 / 10. One 25kg bag covers about 10 m² at 2mm (British Gypsum). Skimming the walls and ceiling of a 4 x 3.5m room with 2.4m walls is 50 m², so 6 bags.

The formula

  • Area (m²) = total wall length x height + ceiling area
  • Multi-Finish or Board Finish bags (25kg) = area x 1.1 / 10

Worked example

Skimming the walls and ceiling of a 4 x 3.5m room, 2.4m high:

  • Walls: 15m x 2.4m = 36 m²; ceiling 14 m²; total 50 m²
  • Bags: 50 x 1.1 / 10 = 5.5, so 6 bags of Multi-Finish
  • Walls only: 36 x 1.1 / 10 = 4.0, so 4 bags

Work it out for your job

Change the measurements and the quantities update straight away.

A 4 x 3.5m room is 15m. Enter 0 for a ceiling only.
Area to skim
50 m²
Multi-Finish
6 x 25kg (+10%)

Your list

  • 6Multi-Finish plaster, 25kg bags (10 m² each at 2mm)
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Materials checklist

  • Multi-Finish plasterFor skimming plasterboard, bonded surfaces and most undercoats. About 10 m² per 25kg bag at 2mm.
  • Board Finish plaster (alternative)Made for plasterboard and low-suction surfaces. Same 10 m² per bag at 2mm.
  • Scrim tapeOver plasterboard joints before skimming.
  • Bonding agentThistle Bond-it for smooth, low-suction or painted walls.
  • Corner and stop beadsFor outside corners and edges.

You'll also need

  • Plastering trowel, hawk and a bucket trowel
  • Mixing paddle and drill, and two clean buckets
  • Dust sheets and a stiff brush
  • Mist coat emulsion for decorating afterwards

Step by step

  1. Prepare the surfaceRemove loose material, tape plasterboard joints and treat the background so it bonds.
  2. MixAdd plaster to clean water and mix to a smooth, lump-free paste. Mix only what you can use in the working time.
  3. First coatApply a thin, even coat and flatten it.
  4. Second coatAs it firms up, apply the second coat to bring the total to about 2mm.
  5. Trowel upTrowel the surface smooth as it sets, then let it dry fully before a mist coat.

Tips from the manufacturers

  • British Gypsum applies Multi-Finish in two coats to a total of about 2mm, and gives about 10 m² per 25kg bag at that thickness.
  • On smooth, low-suction or painted surfaces, British Gypsum recommends Thistle Bond-it, and says it is the only bonding agent it recommends with Thistle gypsum plasters (except HardWall). On very high-suction walls, it recommends Thistle GypPrime.
  • Undercoat plasters such as BondingCoat or HardWall go on about 8 to 11mm thick and cover far less, roughly 3 m² per 25kg bag. Check the data sheet for the one you use.
  • Paint new plaster with a mist coat (thinned emulsion) once it has dried out completely.

Mistakes to avoid

  • Mixing a whole bag at once on your first job and having it set before you finish.
  • Using dirty buckets or water, which makes the plaster set too fast.
  • Skimming over dusty or very absorbent walls without treating them, which can cause cracking or poor adhesion.
  • Painting full-strength emulsion onto new plaster.

Frequently asked questions

How much does a bag of Multi-Finish cover?

About 10 m² per 25kg bag at the normal 2mm thickness, according to British Gypsum.

How many bags of plaster to skim a room?

The walls and ceiling of a 4 x 3.5m room (50 m²) need 6 bags including 10%. A 3 x 3m bedroom with 2.4m walls and the ceiling (about 38 m²) needs 5.

Should I use PVA before skimming?

British Gypsum recommends Thistle Bond-it on smooth or low-suction surfaces and says it is the only bonding agent it recommends with Thistle gypsum plasters. On very absorbent walls it recommends Thistle GypPrime.

How thick is a skim coat?

About 2mm, applied in two thin coats.

Multi-Finish or Board Finish?

Board Finish is designed for plasterboard and low-suction backgrounds. Multi-Finish suits plasterboard and most undercoat plasters too, so it is the usual choice for DIY.

Sources

Quantities are estimates for typical jobs, using the coverage figures above. Products vary, so check the data sheet for the product you buy, and talk to a professional for structural, electrical or gas work. SmartBuild is independent and not affiliated with any of the manufacturers listed.

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