Limestone walls: why the wrong paint destroys them

Limestone is one of the defining materials of Perth's older coastal suburbs — Cottesloe, Fremantle, Mosman Park, parts of Nedlands. It is beautiful, it is durable, and it is routinely ruined by being painted with the wrong thing.

Why limestone is different

Limestone is porous and it works as part of a moisture system. Ground moisture and rain move into the stone and evaporate back out through its face. That cycle is normal and the material tolerates it, provided the moisture can leave as easily as it arrived.

Salts travel with that moisture — from the ground, from the sea air, from the mortar. As water evaporates at the surface, the salts crystallise there and are washed away by rain. The stone stays in balance.

What a standard acrylic does

An ordinary exterior acrylic forms a low-permeability film. On render over brick that is fine. On limestone it is a trap.

Moisture still enters the wall, from the ground or from behind. It can no longer evaporate through the face. It moves to the boundary between the stone and the coating, and the salts crystallise **there**, under the film, instead of on the surface where rain could remove them.

Crystal growth exerts real pressure. It pushes the coating off, and it takes the face of the stone with it. The result is spalling — the surface of the limestone lifting away in flakes and leaving a soft, crumbling face behind.

What to use instead

The requirement is **high permeability** — a coating that lets water vapour out while shedding liquid water.

**Limewash** is the traditional answer and still the most compatible. It is highly permeable, it carbonates into a bond with the stone rather than sitting on it, and it weathers by gentle erosion rather than by peeling. It needs more frequent renewal and its appearance changes with weather, which some people love and some do not.

**Mineral silicate paints** are the modern engineered version. They chemically bond with a mineral substrate, are extremely permeable, and last considerably longer than limewash. More expensive, and they need a mineral substrate to bond to — they will not go over an existing acrylic.

**Highly permeable mineral-modified acrylics** are the compromise. Easier to apply, more colour choice, and considerably more breathable than a standard acrylic without being a true silicate.

The best answer is often nothing

Sound, clean limestone does not need coating. It is a weathering material that has done its job for a century unpainted. Washing it and repointing failed mortar will usually do more for its appearance than any coating, without adding a maintenance liability.

The strong recommendation on unpainted limestone is: do not start.

If it has already been painted

You have three options, and the right one depends on condition.

1. **Leave it and maintain it.** If the existing coating is sound and the wall is dry, recoating with a compatible product is the least risky path.
2. **Remove it.** Possible with poulticing or careful low-pressure methods. Slow, expensive, and results vary with how deeply the paint penetrated. Never sandblast — it destroys the surface faster than the paint did.
3. **Switch to a breathable system as the old one fails.** Where the acrylic is already lifting, removing what is loose and moving to a mineral system lets the wall start drying again.

Mortar matters too

Original limestone walls were built with **lime mortar**, which is softer than the stone by design — it takes the movement and it can be replaced. Repointing with modern cement mortar is the same error as sealing the face: it is harder than the stone, so movement and salt damage are transferred into the stone instead of the joint.

If someone is repointing your limestone, ask what mortar they are using before they start.