Painting exterior masonry is a commitment. The paint system you choose determines how long the maintenance cycle is, how the building breathes, and whether the masonry beneath the paint is protected or damaged. The most common choice — standard exterior latex — is almost always the wrong one. It is the wrong permeability, the wrong durability, and the wrong bond for most masonry substrates.
Correctly painted masonry looks like painted masonry — the texture of the substrate reads through the paint, the surface is matte or near-matte, and the paint does not appear to sit on top of the surface. Incorrectly applied masonry paint looks like a coating applied over a substrate — the texture is obscured, the surface is slightly shiny, and the paint peels.
The correct paint for exterior masonry on traditional New England buildings is mineral silicate paint — Keim Mineral Paint is the standard product. Mineral silicate paint bonds chemically to the calcium silicate in masonry rather than forming a film on the surface. It is highly vapor-permeable, UV-resistant, and extremely durable — typical service life of 20 to 30 years. Elastomeric coatings are the alternative for buildings with known moisture infiltration issues where bridging of surface cracks is the specification.
Keim Mineral Paint does not peel. It weathers by slowly chalking — becoming thinner — rather than by lifting and peeling the way latex does. It does not trap moisture in the wall. It is the only exterior masonry paint that is truly appropriate for old masonry with inherent moisture movement.
Standard exterior latex on old masonry peels. It is too impermeable for masonry that cycles through wet and dry states. The moisture trapped behind the latex film causes it to lift and peel within a few years. Mineral silicate paint, being vapor-permeable, allows the wall to behave as it must.
Keim Mineral Paint through specialty architectural coatings distributors. It is not available at hardware stores. The product requires a specific primer (Keim Fixativ) and specific surface preparation. Work with a painting contractor who has mineral silicate paint experience.
Keim Mineral Paint over Keim Fixativ primer for all exterior masonry painting on traditional New England buildings. No standard exterior latex on historic masonry. No film-forming coatings on rubble stone or porous brick that must breathe. The paint must be more permeable than the wall it covers.
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