Budget 3-5 m² per litre on the first coat over bare lime plaster, then 8-12 m² per litre on coats two and three. Sealed or previously painted lime sits in the normal 10-14 m² band. Three coats is the usual finish on new plaster.
On this page
- Quick recap
- The coverage figure that actually applies to your wall
- Why the label figure assumes a sealed wall
- Run the mist test before you open the paint
- What to do with a wall that beads water
- Should you thin the first coat, and by how much
- How far a litre really stretches
- Plaster age and carbonation change everything
- How to tell if the wall is still curing
- The sum from wall area to litres to order
- Estimating a whole room in one pass
- Signs mid-wall that the coverage figure is wrong
- What to do if you've already lost the coverage battle
- Quick questions
Part of our guide on voc emission standards for flooring
Quick facts
- First coat, bare lime
- 3-5 m² per litre
- Second and third coat
- 8-12 m² per litre
- Sealed lime plaster
- 10-14 m² per litre
- Coats typically needed
- 3 on new, 2 on sound
- Minimum plaster age
- 4 weeks old
- Mist test flash-off
- under 10 seconds is high suction
Quick recap
- First coat on bare lime plaster covers 3-5 m² per litre, not the 10-14 m² on the tin.
- The mist test decides your dilution: water that flashes off in seconds means a thinned first coat.
- Order for the absorbent coat, not the average, or you'll be back at the trade counter mid-wall.
- Two or three coats is normal on lime. A fourth rarely adds anything except time.
- Patchy finish and a dragging brush mean the wall is still drinking harder than you planned.
The coverage figure that actually applies to your wall
Read the coverage band on the tin as a guide for a sealed wall, not yours. Casein paint label figures of 10-14 m² per litre assume a substrate that has stopped pulling. A bare lime plaster wall does not stop pulling on the day you open the tin.
Your first coat on unsealed lime plaster lands around 3-5 m² per litre. Lime is porous and slightly alkaline. The binder soaks in with the water instead of sitting on the surface, so much of that first litre never becomes film. The figure holds when the wall is bare, absorbent and at least four weeks old.
The number depends on three things. First, how open the plaster is. Second, how much you thin the coat. Third, how wet the wall feels on the day. A damp-skimmed wall covers better than a dusty, bone-dry one. So does plaster that has already taken a mist coat of the same paint.
If your wall was limewashed or painted before, work on 10-12 m² per litre and treat it as a different job. The suction is gone and the paint behaves. The rest of this page is about the hard case: bare, hungry lime. We go through silicate paint over lime plaster compatibility step by step elsewhere on the site.
That's the gap nobody else fills. Order in the 3-5 band for coat one and you won't run short at the far end of a gable wall.
Why the label figure assumes a sealed wall
Manufacturers test on a non-porous panel or a primed board. The paint forms a film and stays there. Lime plaster wicks the carrier water away in seconds and takes the binder with it.
Seal the wall with a primer or a thinned first coat and you move onto the label figure. Skip that and the first coat pays for the absorption. If that sounds like your situation, read up on lime plaster wall cracks with clay render next.
Run the mist test before you open the paint
Spray a patch the size of a dinner plate with plain water from a plant mister. Watch the surface. What happens in the next thirty seconds tells you which coverage band you're in.
Water that disappears in under ten seconds means high suction. Mix your first coat 20-30% water. Water that sits as beads for a minute or more means the wall is closed or greasy. Clean it and expect standard coverage, but check for old distemper first.
Water that darkens the plaster and sinks in over about thirty seconds is the middle case. Thin the first coat 10-15% and work on 4 m² per litre. Do the test in three places: a sunny wall, a shaded corner and near a skirting. Absorption varies across a room. For the detail, see our notes on selecting plaster mix to avoid damage.
The mist test costs nothing and takes two minutes. Skipping it is the mistake that turns a one-day job into two. A plastic pump sprayer and a cheap moisture meter earn their keep on a whole-house job, but the mister and your eyes do most of the work here.
Run the test after the wall has had its four weeks. Testing a wall that's still curing wastes your time.
What to do with a wall that beads water
Beading means the surface is sealed or dirty. Wash with a stiff brush and warm water, let it dry, then re-test. If it still beads, you're painting over a previous finish and normal coverage rules apply. It helps to understand avoiding off-gassing from primers before going further.
Spray Bottle for Plants 2 Pack
Gives you a true fine mist for damping the lime plaster and running your suction test on a patch, with 500 ml per bottle so two bottles cover a test day and a spare.
Should you thin the first coat, and by how much
Yes, thin the first coat. Lime plaster grips a soak-in coat better than a thick film. A thinned coat wets the surface, keys into the pores and gives the second coat something to sit on.
The trade-off is coverage. Adding 25% water takes your effective spread from roughly 5 m² per litre down to about 3.5. That's the deal you accept for a bond that holds.
Mix by volume, not by eye. One part water to four parts paint gives a 20% dilution. Stir it through and let it stand for ten minutes. Casein settles, so stir again before you pour.
Don't thin coats two and three. They cover at 8-12 m² per litre at full strength and build the surface. Thinning them wastes pigment and leaves a chalkier finish.
On very dry, dusty plaster, damp the wall with a fine mist first. This slows the pull and buys you working time. Don't soak it. Damp, not wet.
| Coat | Dilution | Coverage on bare lime | Purpose |
|---|---|---|---|
| First | 20-30% water | 3-5 m² per litre | Seal and key |
| Second | None | 8-12 m² per litre | Build colour |
| Third | None | 8-12 m² per litre | Even out finish |
| On sealed lime | None | 10-14 m² per litre | Standard regime |
How far a litre really stretches
One diluted litre covers a patch about 2 metres by 2 metres on bare lime. That's a single doorway-sized panel. Measure your wall before you guess.
Plaster age and carbonation change everything
Fresh lime plaster cures by carbonation. It pulls carbon dioxide from the air and slowly turns back towards limestone. That process takes weeks and it changes how the surface behaves.
Paint before the wall has carbonated and you trap moisture under a film. Casein is breathable, so it's more forgiving than acrylic, but early paint still risks a soft, dusty surface and a patchy finish. Wait at least four weeks on new lime plaster. Six to eight for thick or damp-prone walls.
Older, well-carbonated plaster absorbs less. A wall that's been up for years and never painted sits in a narrower band near 5 m² per litre on the first coat. Change is slow after that.
The risk of painting too early is real and expensive. Blown patches and lifting paint mean scraping back to bare plaster and starting the cycle again. That's a week of your life you won't get back.
Check the wall first. It should be a uniform pale grey with no dark damp patches. If it's still mottled and cool to the touch, it's not ready.
How to tell if the wall is still curing
Press the back of your hand to the wall. A curing wall feels cool and slightly damp. A ready wall feels dry and room-temperature. Use a moisture meter for a final check.
The sum from wall area to litres to order
Work in this order and you'll order the right amount first time. Measure the wall in metres, height times width, and add up the panels. Subtract the doors and windows.
Divide the total area by 4. That's your first-coat litres at the mid-point of the 3-5 band. Divide the same total by 10 for coat two and again for coat three. Add the three figures together.
Round up to the nearest litre and add one spare. Casein paint can't be matched once it's dry and a mid-wall shortage costs you a trip and a lap mark.
Example: a wall 4 m by 2.5 m with a 1 m² window is 9 m². First coat: 9 ÷ 4 = 2.25 litres. Second: 9 ÷ 10 = 0.9. Third: 0.9. Total 4.05 litres. Buy 5.
Work each wall or room separately, then add. Absorption on a south-facing gable is different from a shaded internal wall. Averaging the whole house hides the hungry bits.
Buy plain water-based casein without acrylic or PVA binders. Casein needs the surface to stay breathable, and a polymer additive closes it. That's the one specification worth holding to.
| Stage | Area ÷ divisor | Litres |
|---|---|---|
| First coat | 9 ÷ 4 | 2.25 |
| Second coat | 9 ÷ 10 | 0.90 |
| Third coat | 9 ÷ 10 | 0.90 |
| Round up and spare | total 4.05 → 5 | 5 |
Estimating a whole room in one pass
Add the lengths of all walls, multiply by the height for total area, then subtract 2 m² for each door and 1 m² for each window. Divide by 4 for first coat litres.
Signs mid-wall that the coverage figure is wrong
Watch the brush, not just the wall. A brush that starts dragging and chattering over the plaster is telling you the wall is drinking faster than your plan. Damp the next section lightly and slow down.
Patchy, uneven colour with lighter areas means the coat is too thin there. Go over it before it dries. Casein blends wet-on-wet and won't blend once dry.
Bare spots that look chalky and untouched after a full coat mean you thinned too much or the wall was dustier than the mist test suggested. A second pass on that patch fixes it.
Lap marks that show up hours later mean you stopped and started on a dry edge. Work in wet-edge sections of one to two metres and keep a clean cut line.
If the whole wall is fighting you, stop and re-test. The first instinct to add more coats rarely helps. The right fix is usually a thinned sealing coat and a fresh start on the next section.
One small tin of the same casein is worth carrying for touch-ups. Chalky wall edges scuff and a litre of the right mix saves a whole-wall repaint.
What to do if you've already lost the coverage battle
Stop. Let the wall dry fully, mist it lightly and start the next section with a properly thinned first coat. You'll lose half a day but save the finish.
Casein is the right paint for unsealed lime plaster, but you must budget for a hungry first coat at 3-5 m² per litre and run the mist test before you mix.
Quick questions
How many coats of casein paint does bare lime plaster need?
Three is the usual count on new plaster. The first seals and keys, the second builds colour, the third evens out. Two can be enough on plaster that has already been painted or limewashed and still holds a clean surface.
Can I use casein paint on a wall that's still damp?
No. Casein breathes, but damp trapped under the film still blows the finish. Let the wall dry to a uniform pale grey before you start. A moisture meter reading taken across the wall gives you a firm answer.
Why did my litre cover only half the wall?
Your wall is in the high-suction band. Thinning takes the spread down and a dry, dusty wall pulls hard. That's normal on bare lime. Budget from the 3 m² per litre end next time.
Does the mist test work on old plaster too?
Yes, and it's worth doing every time. Old plaster can be sealed in one corner and hungry in another. Testing three spots catches the variation before it costs you a coat.
Is casein paint a good choice for lime plaster?
It's one of the better matches. Both are breathable and vapour-open, which suits solid walls, especially older UK stock. The trade-offs are a shorter pot life and a chalkier finish than acrylic.
