Painting a 4.2 × 3.4 m bedroom after plastering
One real room, freshly skimmed, and the decision that changes the order more than any other: whether you mist coat it first. The answer is not the one most people expect — skipping it costs you more paint and more money.
The room
- 4.2 m long × 3.4 m wide, 2.4 m floor to ceiling
- One standard door, one standard window
- Walls and ceiling both freshly plastered and dry — new skim, never painted
- Skirting, door, frame and window reveals all being painted white
- Two coats of colour on walls and ceiling
Step 1 — the area, and what comes off it
Perimeter is 2 × (4.2 + 3.4) = 15.2 m. At 2.4 m high that is 36.5 m² of gross wall.
A standard door is 1.68 m² and a standard window 1.44 m², so 3.12 m² comes off, leaving 33.4 m² of wall. The ceiling is 4.2 × 3.4 = 14.3 m².
Total to cover, per coat: 47.6 m².
That 3.12 m² deduction is worth about 8% of the wall area, which is most of a litre across two coats. It is also the deduction people skip because it feels like a rounding error, and then wonder why they have a third of a tin left over.
Step 2 — the mist coat, which is not really about the mist coat
New plaster is porous. It pulls the liquid out of the first coat you put on it, so that coat covers less — the coverage table puts sealed new plaster at 10 m² per litre, with the first coat achieving ×0.75 of that on an unsealed surface.
So the room can be painted two ways, and the difference is larger than it looks:
| Mist coat first | Straight onto plaster | |
|---|---|---|
| Coverage, coat one | 10 m²/L | 7.5 m²/L |
| Coat one | 4.76 L | 6.35 L |
| Coat two | 4.76 L | 4.76 L |
| Emulsion needed | 9.53 L | 11.1 L |
| Tins you can actually buy | 1 × 10 L | 1 × 5 L + 1 × 2.5 L + 4 × 1 L |
| Emulsion purchased | 10.0 L | 11.5 L |
The litre difference is 1.6 L. The cost difference is much worse than that, because of the right-hand column's last row. One 10 litre tin is the cheapest emulsion per litre you can buy. Four 1 litre tins is close to the most expensive. You have not just bought more paint, you have bought it in the worst possible format — and you still have to buy the mist coat's worth of paint eventually if you ever want the finish to be even.
A mist coat is thinned emulsion, not primer. Ordinary matt emulsion cut roughly 70:30 with water. You do not need a special product, and a "plaster sealer" is usually the same thing at four times the price. What you must not use is anything vinyl or silk — it seals the surface without keying to it, and the topcoat peels in sheets a year later.
This room takes 4.33 L of mist coat at 11 m² per litre. That is 3 L of emulsion and about 1.3 L of water, which comes out of the 10 L tin you already bought.
Step 3 — trim, which is bigger than it looks
Trim gets left off the order more often than anything else, because it does not feel like an area. It is:
- Door: 2 × 1.68 m² for both faces, plus 0.55 m² for frame and architrave = 3.91 m²
- Window: reveals, cill and beading, about 35% of the opening = 0.50 m²
- Skirting: 15.2 m around the room at an effective 0.15 m band = 2.28 m²
6.7 m² in total — a seventh of the ceiling, from three things that look like nothing. At 14 m² per litre for joinery, two coats is 0.96 L. Buy a 1 litre tin and expect to use almost all of it.
Step 4 — the full order
| Item | Qty | Why this much |
|---|---|---|
| Matt emulsion, 10 L | 1 tin | 9.53 L for two coats; the mist coat is thinned out of the same tin |
| Trim paint, 1 L | 1 tin | 0.96 L over 6.7 m² of joinery, two coats |
| Wood primer, 750 ml | 1 tin | Bare skirting and frame only. Skip if the trim is already painted and sound. |
| Roller sleeves, medium pile | 3 | One for mist coat, two for colour. A mist-coat sleeve is ruined. |
| Roller frame, 9 in, and pole | 1 | The pole is not optional for a 2.4 m ceiling |
| Brushes, 50 mm and 25 mm | 1 each | Cutting in, and the window beading |
| Paint tray and liners | 1 + 4 | Liners save washing out between colours |
| Filler, ready-mixed | 1 tub | Nail holes and the shrinkage crack that appears at the ceiling line |
| Caulk, 310 ml | 2 cartridges | Skirting-to-wall and frame-to-wall, about 22 m of line |
| Masking tape, low-tack | 1 roll | Low-tack specifically — ordinary tape lifts fresh emulsion |
| Dust sheets | 2 | Cotton, not plastic. Plastic holds drips wet and you walk them through the house. |
| Sandpaper, 120 and 180 grit | 1 pack each | Between trim coats, and to knock back nibs on the skim |
What people forget
- That the plaster has to be dry, not just touch-dry. Fresh skim needs weeks, not days — longer in winter, longer again if the room has no heating on. Painting damp plaster traps the moisture and the finish blisters. If it is still darker in patches, it is not ready.
- That the mist coat comes out of the same tin. People buy 10 L of colour and then a separate 5 L of "sealer" they did not need. Thin the emulsion you already have.
- The ceiling line crack. New plaster shrinks slightly as it cures, and the wall-to-ceiling junction is where it shows. Caulk it after the mist coat, not before — you want to see where it actually opened up.
- Third-coat risk on strong colours. Two coats is the honest figure over a white mist coat. A deep red, a strong blue or a yellow over white will very likely want three, and the tin will still say two.
- Buying trim paint by the 2.5 L tin. You need one litre. Trim paint skins over in a part-used tin faster than emulsion does, and you will throw most of it away.
What this costs in time
Two days of actual work, spread across three because of drying:
- Day 1 — fill, sand, mist coat walls and ceiling. The mist coat dries fast; it is thin.
- Day 2 — caulk the ceiling line and skirting, prime bare trim, first coat of colour on walls and ceiling.
- Day 3 — second coat of colour, then two coats on the trim last, so drips land on paint you are not going back to.
Trim last is worth saying twice. Doing the woodwork first and then rolling the walls is how the skirting ends up speckled.
Run your own version
Every figure above came out of the calculator with this room's dimensions typed in, the surface set to new plaster, and the mist coat box ticked:
- Paint calculator — tick and untick the mist coat to see the tin plan change
- Paint coverage rates — where 10 m²/L and the ×0.75 come from
- How to measure a room properly — before anything else
- Plasterboard calculator — if the room is not plastered yet
Last reviewed 2026-10-17. Quantities are for the room described. Your room will differ — run the numbers rather than copying the totals.