Paintable surfaces include masonry, wood, metal, plastic/vinyl, and composites. In the case of masonry paint, the basic principle is the same, protection and decoration of a masonry surface. A masonry surface is generally considered concrete, brick and mortar, stucco, cement block, and sandstone. Because masonry ordinarily presents a much more porous surface than, for example, wood or metal, masonry primers and paint must be formulated to effectively counter porosity.
Additionally, masonry, because of its susceptibility to moisture absorption and retention, is not an especially good insulating material. Thus, masonry paint must provide an insulating value to the substrate. Breathability of the paint is crucial to the effectiveness of the masonry coating in allowing trapped moisture to escape outside the surface while preventing more moisture from entering.
Masonry paint may be an oil-based, or alkyd, coating, or it may be latex, or water-based. Commonly, a masonry primer will be oil-based, and the topcoat will be latex. Home masonry do-it-yourselfers note: a latex product may be applied over an oil-based product, but the reverse is not a good idea.
Masonry surfaces present a number of unique surface-preparation challenges including flaking, dust, and efflorescence, or chalking, as well as the common problem of expansion and contraction. Proper preparation of the masonry surface is very important to the effectiveness of both oil-based or latex masonry primers and paints. Products specifically designed to clean and etch masonry surfaces must be properly applied, while cracks, deterioration, and defects in the surface must be repaired before painting.
Numerous manufacturers of masonry paints and primers boast of the addition of poly vinyl acetates (PVA) to the product to increase adhesion properties. Others claim added stabilizers, or binders designed to hold together flaking, crumbling, and dusty masonry that characterize older, deteriorating surfaces. Some masonry paint manufacturers offer both additives together. However, there seems to be some contention as to how effective these stabilizing and binding compounds can be, and whether they present more long-term problems than solutions.
In simplistic terms, then, masonry paint is a coating and sealant that must be elastic, breathable, and durable. It must not dry too quickly, remaining wet enough to be brushed, rolled, or sprayed directly onto properly prepared masonry with a uniform consistency. Assuming the proper masonry preparation and priming has been done, then any quality exterior paint, being essentially a weatherproof paint, will work as masonry paint.