Lime plaster is made by mixing lime, sand, and water in specific ratios, then applying it in three thin coats that each need days to cure. The process is straightforward but unforgiving if you rush it. Getting the materials, mix ratios, and curing conditions right is the difference between a plaster that lasts centuries and one that cracks within weeks.
Choosing Your Lime
The first decision is which type of lime to use, and it depends on where you’re plastering. There are two broad categories: non-hydraulic lime (sold as lime putty or hydrated lime powder) and natural hydraulic lime (NHL). Non-hydraulic lime sets purely by absorbing carbon dioxide from the air, a slow process called carbonation. Hydraulic lime sets partly through a chemical reaction with water, similar to cement, which makes it faster and more weather-resistant.
For interior plastering, non-hydraulic lime putty gives the smoothest, most breathable finish. It’s the traditional choice for old buildings and conservation work. For exterior walls or anywhere exposed to moisture, hydraulic lime is the better option.
Hydraulic lime comes in three grades. NHL 2 is the softest and slowest setting, ideal for interior plaster and for use on fragile, crumbling stone or brick. NHL 3.5 is the general-purpose grade, suitable for most brick or stone walls with moderate weather exposure. NHL 5 is the strongest and fastest setting, reserved for dense masonry in severe conditions like chimneys, plinths, and coastal structures. For most plastering projects, NHL 2 or NHL 3.5 is what you want. NHL 5 is too hard for typical walls and can actually damage softer masonry by trapping moisture.
The Basic Mix Ratios
Lime plaster uses a simple formula: lime plus sand plus water. The ratio changes slightly depending on the coat you’re applying.
- Scratch coat (first coat): 1 part lime to 2.5–3 parts sharp, well-graded sand. This is the coarsest layer, and the sand should include a range of grain sizes up to about 3–4 mm.
- Float coat (second coat): 1 part lime to 2.5–3 parts sand, using a slightly finer sand than the scratch coat.
- Finish coat (third coat): 1 part lime to 2–2.5 parts fine sand. Some plasterers use lime putty alone or with a very fine silver sand for the final skim.
Measure by volume, not weight. Mix the dry lime and sand together thoroughly before adding water gradually. You’re aiming for a consistency similar to thick peanut butter for the base coats, and slightly wetter and creamier for the finish coat. If you’re using lime putty rather than powder, it already contains water, so you’ll need less added liquid. The mix should hold its shape on a trowel without slumping but spread easily without tearing.
Adding Fiber for Crack Resistance
The scratch coat and float coat benefit from fiber reinforcement, which holds the plaster together while it cures and reduces cracking. Traditionally, plasterers used goat or horse hair cut to roughly 25–50 mm lengths. You can still buy animal hair for this purpose from lime suppliers. Pull it apart and tease out any clumps before mixing it in.
Modern alternatives include chopped polypropylene, sisal, or kenaf fibers. Research comparing these options found that natural fibers like sisal and kenaf perform comparably to synthetic polypropylene in lime plaster reinforcement. You only need a small amount, around 0.2% of the mix by weight, which works out to a loose handful per bucket of plaster. Add the fibers to the dry mix and distribute them evenly before adding water. The finish coat is typically applied without fibers for a smooth surface.
Preparing the Wall
Lime plaster bonds mechanically, meaning it grips onto a rough surface rather than chemically fusing to it. The wall needs to be clean, free of loose material, and dampened before you start. Dry masonry or old plaster will suck the water out of your mix too quickly, causing it to crack and fail.
Spray the wall with clean water using a garden sprayer or brush. You want the surface damp but not running wet. On very porous backgrounds like soft brick or stone, you may need to dampen the wall several times in the hours before application. If you’re plastering onto wooden lath, the lath should be riven (split, not sawn) for the best key, and spaced about 8–10 mm apart so the plaster can squeeze through the gaps and curl behind to lock itself in place.
Applying the Three Coats
Lime plaster is always built up in thin layers. No single coat should exceed 15 mm thick, and thinner is generally better.
The scratch coat goes on first at about 10 mm thick. Press it firmly onto the dampened wall with a steel trowel, working from bottom to top. Once it starts to firm up but is still soft enough to mark, scratch horizontal lines across the surface with a comb or the edge of a lath. These grooves give the next coat something to grip. Allow roughly 10 days of drying time before applying the next layer, though this varies with temperature and humidity. In cool or damp conditions, it can take considerably longer.
The float coat, also called the brown coat, is the second layer. Dampen the scratch coat before applying it. This coat goes on at about 10 mm thick and gets leveled with a straight edge or darby to create a flat, even plane. Don’t over-trowel it. Work it just enough to flatten and then leave it alone. Excessive troweling brings lime to the surface and weakens the coat. Scratch it lightly to key for the finish coat, and again allow about 10 days to cure.
The finish coat is the thinnest layer, applied at about 4 mm. This is where you determine the final texture. For a smooth finish, apply the lime-rich mix with a clean steel trowel and compress it with light, sweeping strokes as it firms. For a more rustic look, you can apply it with a wooden float and leave it with a slightly open texture. The finish coat needs roughly 4 days to set firm, though full carbonation takes much longer.
Curing Conditions Matter
This is where most beginners run into trouble. Lime plaster does not cure like cement. Non-hydraulic lime sets by carbonation, absorbing CO2 from the air, and the rate depends heavily on humidity. Research measuring carbonation under controlled conditions found that lime paste exposed to high humidity (around 97% relative humidity) carbonated nearly twice as fast as paste kept at 65% relative humidity. The ideal range sits around 75%, which provides enough moisture to keep the chemical reaction moving while allowing the surface to dry gradually.
In practical terms, this means you need to keep lime plaster damp but not soaking wet as it cures. Mist the surface lightly with water once or twice a day for the first week or so, especially in warm or dry weather. If the plaster dries out too fast, it will crack and won’t develop full strength. If it stays waterlogged, carbonation stalls. On exterior work, protect fresh plaster from direct sun, wind, and rain with damp hessian (burlap) sheeting.
Temperature matters too. Lime plaster should not be applied when temperatures are below 5°C or when frost is expected within the curing period. Cold weather slows setting dramatically, and frost can destroy fresh plaster entirely. The ideal working conditions are mild, overcast days with moderate humidity.
Full carbonation of a lime plaster wall takes months, and in thick sections, it can take years for the core to fully convert. The surface will feel firm within days to weeks, but the plaster continues to gain strength slowly over time. This is normal and actually one of lime’s advantages: it remains slightly flexible rather than becoming rigid and brittle like cement-based plaster.
Safety Precautions
Wet lime plaster has a pH of 12.4, which is highly alkaline and caustic enough to cause chemical burns on skin and serious eye damage. This is not a material you can handle casually.
Wear nitrile gloves, long sleeves, and full-length trousers that close at the cuffs to prevent plaster getting inside your clothing. Eye protection is essential: use tight-fitting goggles, not open safety glasses. When mixing dry hydraulic lime powder, the dust is an additional hazard. Wear a dust mask rated at minimum FFP2 for moderate exposure. Keep a bottle of clean water or eyewash nearby at all times. If lime gets in your eyes, flush immediately with water for at least 15 minutes.
If you’re working with quicklime rather than pre-slaked lime putty, the safety stakes are higher. Quicklime reacts violently with water and can generate temperatures exceeding 250°C at hot spots during slaking. This is a specialist process best left to experienced lime workers. For most plastering projects, buying pre-made lime putty or bagged hydraulic lime powder is far safer and more practical.
Storing and Reusing Lime Plaster
One of lime plaster’s practical advantages over cement is its shelf life. Hydraulic lime mixes must be used within a few hours of mixing, since they begin to set chemically with water. But non-hydraulic lime putty plaster can be stored almost indefinitely as long as it stays covered with a thin layer of water to prevent air contact. Traditionally, lime putty was aged for months or even years before use, which improves its workability and produces a finer, creamier plaster. If you mix too much non-hydraulic plaster, just cover it with water in a sealed bucket and use it next time.