Lime paint makes the wall hostile to mould.

The binder in this paint is calcium hydroxide, lime matured under water. Water carries it into the pores of an absorbent mineral wall. As the water leaves, carbon dioxide from the air turns the calcium hydroxide back into calcium carbonate, the mineral limestone is made of, and the mineral the wall’s lime plaster is built from. The paint does not dry into a film. The cured coat is calcium carbonate, the same mineral as the lime plaster under it. This hardening continues slowly for several weeks after the final coat.

The cured surface stays open-pore. Water vapour from bathroom steam, kitchen condensation, and monsoon humidity passes through it in both directions. The figure for this is 0.01, meaning the coat holds back water vapour about as much as a centimetre of still air. An acrylic film is far less permeable. How much less depends on the product.

The second consequence is alkalinity. Calcium hydroxide cures at a pH of about 13, strongly alkaline. That keeps the surface mould-resistant while it cures. The surface is not treated against mould with an added biocide. It is chemically hostile to mould through its own cured composition. The chemistry is the protection.

India’s most common interior paint failure is not colour fade. It is moisture. Bathrooms, kitchens, basement walls, and coastal-city interiors see humidity high enough to test any wall finish. A conventional acrylic emulsion is a sealed polymer film, and a sealed film can trap moisture behind it. Trapped moisture breeds mould, which in turn can rupture the film. Lime paint breaks that chain at both points. The open-pore surface lets water vapour move back out instead of holding it behind a sealed film, and the alkalinity makes mould growth on the surface itself unfavourable.

In Indian interiors, lime paint is typically used in humid bathrooms and kitchens, in heritage havelis and conservation work, and anywhere an architect needs to specify mould resistance without biocide chemistry. In conservation work, lime plaster is already the substrate, and lime paint is the topcoat that matches its chemistry. The 665 TDS states plainly that it suits humid rooms.

Lime paint is the damp-room counterpart to clay paint. Clay buffers indoor humidity by absorbing moisture from the air and releasing it again as the air dries, which makes it the right tool for dry living rooms and bedrooms. Lime resists mould through its high alkaline pH, which makes it the right tool where the moisture load or the mould risk is active. The two mechanisms are complementary, not interchangeable. Match the paint chemistry to the room’s moisture condition.

Lime coatings shed a fine powder as they age. Painters call it chalking, and it is why a whitewashed wall can mark a sleeve. The Indian standard deals with it directly. IS 6278 requires gum in the mix, one kilogram dissolved in hot water for every cubic metre of lime cream, which is the thin lime slurry the standard has the painter prepare. It allows common salt on top of that, about 1.3 kg for every 10 kg of lime. Its own note explains the salt: it makes the lime harden faster, which leaves the coating "hard and rub-resistant". The standard then tests for exactly that: the whitewash "shall not come off readily on the hand, when rubbed".

On site the gum and salt are often replaced by PVA glue. No published measurement of chuna mixed with PVA was found, so the closest evidence is a study of ready-made paints. In 2024 a team tested facade paints sold on the Brazilian market (Buildings 14(4):1016). They coated aerated concrete slabs and measured, by the ASTM E96 desiccant method, how much each coating held back water vapour. The bare slab came out at 8.44. A lime paint, brushed on in a single coat, came out at 8.52. A ready-made PVA paint, thinned by 20% and rolled on in two coats, came out at 10.71.

In plain terms, the lime coat left the slab almost exactly as breathable as it found it, while the two-coat PVA system raised its resistance to water vapour by about a quarter. Neither paint was a LEINOS product, and neither was chuna mixed on site. What the 665 data sheet does state is its own composition, a thickener made from plant cellulose and an organic binder, and the breathability figure in the comparison table below.