Mineral Silicate Paint for London Masonry: Keim, Breathability and Conservation

What Is Mineral Silicate Paint?

Mineral silicate paint is a coating system based on potassium silicate (waterglass) rather than the organic resins used in conventional masonry paints. When applied to a mineral substrate — natural stone, lime render, lime plaster, or unpainted brick — the potassium silicate reacts chemically with the substrate in a process called silicification, forming an insoluble bond that becomes part of the masonry itself rather than sitting as a film on top of it.

This fundamental difference in how the paint bonds to the wall gives mineral silicate systems a set of properties that make them uniquely suited to London's historic masonry — and increasingly popular with conservation officers, architects, and specifiers working on listed buildings, conservation area properties, and heritage restoration projects across the capital.

Why It Matters for London Buildings

London's older masonry — the Portland stone of Westminster, the stuccoed terraces of Belgravia and Pimlico, the stock brick of Victorian suburbs, the lime-rendered facades of Georgian townhouses — was built to breathe. Solid walls, lime mortars, and lime renders allow moisture to move through the building fabric and evaporate from the surface. This moisture management is essential to the long-term health of the structure.

Conventional masonry paints, based on acrylic or silicone resins, form a continuous film over the surface. While modern formulations claim breathability, even the best film-forming paints restrict vapour transmission to some degree. On a solid-walled London building, this can trap moisture within the masonry, leading to damp problems internally, frost damage to the stone or render, and eventual paint failure as moisture pushes the film away from the substrate.

Mineral silicate paint, by contrast, is fully vapour-permeable. Moisture passes through the coating as freely as it passes through unpainted stone or render. The building continues to breathe exactly as it was designed to, and the risk of trapped moisture is effectively eliminated.

The Keim System

The best-known mineral silicate paint system is manufactured by Keim, a German company that has been producing silicate paints since 1878. Keim Granital (for exterior use) and Keim Ecosil (for interiors) are widely specified in London conservation work and have a proven track record on historic buildings across Europe spanning well over a century.

The Keim system typically involves a mineral primer (Keim Fixativ) applied to the prepared substrate, followed by two coats of the silicate paint itself. The primer equalises the suction of the substrate — important on patchy or previously repaired surfaces — and ensures even colour and adhesion of the topcoats.

Keim paints are available in a range of earth-derived pigments that produce soft, natural colours with a characteristic matt, chalky finish. They do not yellow, fade, or chalk in the way that organic paints do, and their colour stability over decades is one of their most valued properties. The finish has a depth and luminosity that is difficult to achieve with conventional paints — a quality that architects and conservation officers consistently cite as an advantage on historic facades.

Where Conservation Officers Specify Silicate Paint

In London's many conservation areas — Belgravia, Mayfair, Bloomsbury, Greenwich, Hampstead, and dozens more — alterations to the external appearance of buildings are controlled by the local planning authority. When a listed building or a property in a conservation area requires external redecoration, the conservation officer may specify or strongly recommend a mineral silicate system, particularly where the existing finish is lime-based or where previous acrylic coatings have caused problems.

Keim paints are specifically approved by English Heritage (now Historic England) for use on listed buildings and scheduled monuments. They are regularly specified for work on churches, public buildings, and landmark residential properties across London.

Preparation and Application

Mineral silicate paint cannot be applied over existing acrylic, vinyl, or oil-based coatings — the silicate must react directly with the mineral substrate. If the masonry has been previously painted with a film-forming product, that coating must be completely removed before a silicate system can be used. This is often the most labour-intensive part of the process, involving steam stripping, careful scraping, or specialist paint removers.

On bare or lime-rendered surfaces, preparation involves cleaning (gentle washing, never high-pressure jet washing on historic masonry), repairing any defective render or pointing with compatible lime mortars, and allowing all repairs to cure fully before painting.

Application is by brush, roller, or spray, depending on the scale and access requirements of the project. The paint handles differently from conventional masonry coatings — it is thinner, dries quickly, and requires a consistent wet edge to avoid lapping marks. Professional applicators experienced with silicate systems achieve the best results, and Keim themselves offer training and technical support for decorators working with their products for the first time.

Cost and Longevity

Mineral silicate paint is more expensive per litre than conventional masonry paint, and the preparation requirements (particularly removing existing coatings) can add significantly to the project cost. However, the longevity of the system is exceptional. Keim-painted facades in continental Europe have remained in good condition for over a hundred years. In the London climate, a lifespan of thirty years or more before recoating is realistic — compared to five to ten years for a typical acrylic masonry paint.

Over the lifetime of a building, the total cost of a silicate system is often lower than repeated repainting with conventional products, and the ongoing protection of the underlying masonry is significantly better. For owners of listed buildings, period properties, and homes in London's conservation areas, mineral silicate paint represents a genuinely superior long-term investment.