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How to Plaster a Room

Which plaster goes on which wall, how to prepare it, the two-coat skim from first trowel to polish, and how long to leave it, with a room planner priced at UK merchant prices, last checked on the date shown.

Page last reviewed 1 November 2026 by Valerii Zugravu · How I check

A freshly skimmed wall is one of the most satisfying finishes in DIY, and one of the easiest to get wrong. The plaster sets on its own timetable, not yours: the art is having the wall prepared, the mix right and the next step ready before it's needed. This guide covers which plaster goes on which wall, the prep, the two-coat skim step by step, and drying. The room planner prices the bags for your room.

Hands applying plaster to a wall with trowels
Skimming is about timing: the right mix, on a prepared wall, trowelled at the right moment.
  • 10m²per 25kg bag of Multi-Finish at 2mm
  • 2 coatsof skim, about 2mm in total
  • ~90 minfrom mixing to final polish (varies)
  • Pale pinkmeans dry: then a mist coat

1. Which plaster goes on which wall?

Three wall build-ups seen end-on: plasterboard gets a 2mm skim straight on; old sound plaster gets diluted PVA then a 2mm skim; bare blockwork gets about 11mm of hardwall undercoat then a 2mm skim Plasterboardskim straight on Old plasterPVA, then skim Bare blockhardwall + skim 2mm skimPVA 1:4~11mm hardwall
Pink is the finishing skim. What goes underneath depends on the wall.
  • Multi-FinishThe all-round skim: boards, old plaster and undercoats.
  • Board FinishMade for plasterboard and low-suction surfaces.
  • HardwallUndercoat for high-suction brick and block, ~11mm.
  • BondingUndercoat for low-suction concrete and painted walls, ~8mm.

2. Prepare the room and the wall

  • ProtectDust sheets down, sockets and switches taped over, everything moved out.
  • Make goodScrape off loose paint and plaster, fill deep holes with undercoat first.
  • Scrim and beadScrim tape over every board joint, angle bead on external corners.
  • Old plaster: PVADiluted about 1:4; skim while it's still tacky, not dry.
  • Plan the sessionOne wall per mix. Start with the ceiling, then walls.
  • No heat or draughtsRadiators off, no fans or direct sun while it sets.

3. Plan and price your room

Enter your room and the wall surface. The planner works out the area after doors and windows, the bags of each plaster, and scrim tape for boards, then prices the plaster at the cheapest UK merchant we've found. It uses the same coverage as the plaster calculator.

m
m
m
m²
A door is about 1.6m², a window 1.5–2m².

The planner needs JavaScript. You can still use the plaster calculator.

Assumes 10% extra, Multi-Finish at 1.25kg per m² per mm and 2mm thick (about 10m² a bag), hardwall at 0.76kg and 11mm, bonding at 0.83kg and 8mm, all in 25kg bags. The ceiling is skimmed only (it's usually plasterboard). Scrim tape allows 1.3m per m² of board. PVA, beads and tape aren't priced.

4. Mixing

  • Clean bucket, clean waterOld plaster in the bucket makes the new mix set faster.
  • Plaster into waterNever water into plaster. About 11.5L for a bag of Multi-Finish.
  • Paddle to yoghurtSmooth, lump-free and thick enough to hold on the trowel.
  • No re-temperingAdding water to setting plaster ruins it. Mix a fresh batch.

5. The two-coat skim, step by step

A typical skim session over about 90 minutes: mix at the start, lay on the first coat in the first 15 minutes, the second coat at around 20 to 35 minutes, then trowel it up three or four times as it firms between about 45 and 75 minutes, finishing with a polish; the exact times depend on the wall's suction and the temperature 0306090 min Mix 1st coat 2nd coat Trowel up as it firms Polish Times are typical: warm rooms and thirsty walls go faster
A typical skim session. The orange marks are trowelling passes as the plaster firms up.
  1. First coat. Load the hawk, lay plaster on from the bottom corner up and across, about 1–2mm thick, pressing it hard into the wall. Cover the whole wall quickly.
  2. Flatten it. Go over the first coat with a lightly angled trowel to knock down ridges. Don't chase perfection yet.
  3. Second coat. As soon as the first coat is firm enough not to drag (often 10–20 minutes), lay on a thinner second coat, filling hollows and trowel marks.
  4. Trowel up. As the plaster firms, trowel the whole wall again with firmer pressure and a flatter blade, three or four times, 10–15 minutes apart. Each pass closes the surface more.
  5. Polish. On the last pass, flick a little clean water on with a brush if needed and trowel until it shines. Stop when your trowel starts to drag or burn.
  6. Clean the tools straight away. Dried plaster on a trowel scratches the next wall.

6. Drying and decorating

  • Dark to pale pinkDry when evenly pale with no dark patches: days for a skim.
  • VentilateOpen windows; gentle heat is fine once it's set, not before.
  • Mist coat firstWatered-down emulsion soaks in and seals it, then paint as normal.

7. Mistakes to avoid

  • Mixing too muchIt sets before you finish. One wall per mix.
  • Over-trowelling earlyWorking it while soft drags ridges. Wait for it to firm.
  • Skipping scrimBoard joints crack through the skim.
  • Skimming onto dry PVASkim while it's tacky, or the bond fails.
  • Painting too soonPaint traps moisture and peels. Wait for pale pink.
  • Heat while settingFast drying causes cracks and a weak, powdery surface.

8. Plastering checklist

Print this page (the planner is left out of the printout) and tick these off as you go:

  • Wall surface identified; plaster and undercoat chosen
  • Plaster, PVA, scrim tape, beads and buckets bought
  • Room cleared, floor sheeted, sockets and switches taped over
  • Loose material scraped off, deep holes filled with undercoat
  • Scrim on board joints, angle bead on corners
  • Old plaster PVA'd and still tacky (if needed)
  • Clean bucket, clean water, one wall per mix
  • First coat, second coat, trowelled up and polished
  • Tools cleaned straight away
  • Left to dry to pale pink, then a mist coat

Painting next? The paint calculator works out the tins, including the mist coat. Building a new wall first? See the stud wall guide.

Frequently asked questions

Can I plaster a room myself?

Yes, but practise first. Skimming is a timing skill more than a strength one: start with a small wall or a spare board, and keep each mix small so you're never racing the set.

How much plaster do I need to skim a room?

A 25kg bag of Multi-Finish covers about 10m² at 2mm. A 4 × 3.5m room with 2.4m ceilings is roughly 32m² of wall after doors and windows, so about 4 bags with a little extra, plus 2 more for the ceiling.

Do I need to PVA before skimming?

On new plasterboard, no: skim it straight on. On old, dry plaster, many plasterers brush on PVA diluted about 1:4 with water and skim while it's still tacky, so the wall doesn't suck the water out of the skim too fast. Follow the plaster maker's advice for your wall.

How long does new plaster take to dry?

A 2mm skim usually dries in a few days to a week, depending on the room and the weather. It's dry when it turns an even pale pink with no dark patches. Thicker undercoats take longer.

Should I use bonding or hardwall?

Hardwall suits high-suction backgrounds like bare blockwork and brick. Bonding suits low-suction surfaces like concrete, engineering bricks or walls treated with a bonding agent. Both then get a 2mm skim on top.

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