Limewash Calculator — Lime Putty, Water & Casein

A minimum of 3 coats is needed for interior walls, with 4 coats being standard. For exterior walls, 5-6 coats are recommended for durability and weather resistance. Each coat should be thin and watery — thick coats will crack and peel. Apply coats in quick succession (same day if possible) while the previous coat is still slightly damp.

The typical ratio is 1 part lime putty to 1-3 parts water by volume. The first coat should be the thinnest (1:3 ratio) to penetrate the substrate. Subsequent coats can be slightly thicker (1:1.5 to 1:2). The consistency should be like semi-skimmed milk. Too thick and it will crack; too thin and it won't build coverage.

Casein is a milk protein that acts as a natural binder, making limewash more durable and water-resistant. A typical addition for exterior use is about 25g of casein powder per litre of limewash — this matches the Tŷ-Mawr Lime rate of 500g per 20-litre bucket (practitioner rates range roughly 20-25g/L). Dissolve the casein in warm water first to form a paste, then mix into the limewash. Casein makes the limewash slightly more weather-resistant but still allows the wall to breathe — and, as the SPAB notes, binders should only be added where there is good justification, as any additive can reduce breathability.

No — limewash is only suitable for lime-based surfaces such as lime plaster, lime render, and natural stone. It will not adhere properly to cement render, gypsum plaster, or modern masonry. The lime needs to carbonate (absorb CO2) into the substrate, which only works on compatible lime surfaces. For cement render, use standard masonry paint instead.

Interior limewash lasts 5-10 years before needing a fresh coat. Exterior limewash lasts 3-7 years depending on exposure. South and west-facing walls weather faster. Unlike modern paint, limewash weathers gracefully — it slowly erodes rather than peeling or flaking. Recoating is simple: just dampen the wall and apply 2-3 fresh coats over the existing limewash.