This page is the practical side: the order of work, what each coat does and how to look after it. For the background on why lime suits old buildings, see our lime plaster explainer.
Step 1: prepare the background
Hack off any failed or loose plaster back to sound material, and remove any gypsum or cement patches, which trap moisture against old walls. Rake out soft joints and brush the wall down to remove dust.
The day before and again on the day, dampen the wall with a mist sprayer. A dry background sucks water out of the lime too quickly and stops it bonding. Wet it until it darkens, but do not leave water running down the surface.
Step 2: dubbing out the hollows
Old walls are rarely flat. Deep hollows are filled first with a coarse lime mortar, sometimes with pieces of tile or stone pressed in, so the main coats can go on at an even thickness. Each dubbing layer is scratched and left to firm before the next.
Step 3: the scratch coat
The first full coat is pressed firmly into the wall to make good contact, laid on roughly a centimetre thick. Once it has stiffened, it is scored with a comb or lath scratcher in a diamond pattern to give the next coat a key.
Leave it to firm up properly. That might be a couple of days or longer depending on weather and the wall, and it must be misted to stop it drying out during that time.
- Press on firmly, do not just skim over
- Score in a criss cross pattern
- Mist regularly while it firms up
- Protect from draughts, direct heat and frost
Step 4: the float coat
The second coat is ruled flat with a straight edge and then worked with a wooden float to close the surface and consolidate it. Timing matters: floating too early drags the material, too late and it will not close up. It is then keyed lightly with a devil float for the finish coat.
Step 5: the finish coat
A thin finish of fine lime putty and sand is applied and trowelled or floated to the texture wanted. A trowelled finish is smooth; a float finish is softer and more traditional. Overworking with a steel trowel can bring lime to the surface and cause crazing, so a light hand is best.
Curing and decorating
Lime hardens by taking in carbon dioxide from the air, which is slow. Keep the plaster damp for several days, shield it from sun, heaters and draughts, and never let it freeze. Hessian hung close to the wall helps in drying conditions.
Decorate only with breathable finishes such as limewash or suitable clay or mineral paints. Vinyl emulsions seal the surface and defeat the point of using lime.
Tools and materials to have ready
Have a mist sprayer, hawk and trowels, a wooden float, a devil float or scratcher, a straight edge, buckets and hessian or dust sheets for curing. Use a well graded sharp sand and the lime specified for the job, and keep mixes consistent from batch to batch so colour and strength match across the wall.
Common mistakes
Most lime failures come from rushing between coats, working onto a dry background, heating the room to speed drying, or patching with gypsum. Each of those leads to cracking, poor bond or trapped damp.
We have over 40 years of experience working in older Kent buildings. If you would rather have it done for you, book a free site visit and we will provide a fixed-price written quote.
For the background, read our explainer: lime plaster explained.
Common questions
Can I put lime plaster over gypsum?
It is not recommended. Gypsum should be removed so the lime can bond to the original breathable background.
How long between coats?
Until the previous coat has firmed up enough to resist a thumb press, which can be a day or several days depending on conditions.
What paint goes on lime plaster?
Breathable finishes such as limewash or suitable mineral or clay paints, not ordinary vinyl emulsion.
Why is my new lime plaster cracking?
Usually because it dried too fast, went on too thick in one coat, or the background was too dry when applied.