Why does paint stain and discolour on masonry?

Q: Why does paint stain and discolour on masonry?

There are a number of causes but the main ones are: Certain types of sand used in the construction or rendering of a building can cause staining, as can certain kinds of brick hollow clay pots or clinker blocks containing soluble salts. Pieces of ferrous metal or iron stone embedded in the material can rust and discolour when the surface is painted. These problems can be treated by sealing the affected areas with good primer, one that resists alkaline. Rust staining can occur where old nails are left in the substrate, or a wire brush has been used to prepare the surface. In this case, prime those areas with a good metal primer. Steel reinforcing in concrete may be too close to the surface and cause rusting. The only effective way to resolve this problem is to get back to the metal itself and treat the cause of the rust then make any necessary repairs to the surface and repaint.

Question

idadm · Accepted Answer

There are a number of causes but the main ones are:

Certain types of sand used in the construction or rendering of a building can cause staining, as can certain kinds of brick hollow clay pots or clinker blocks containing soluble salts. Pieces of ferrous metal or iron stone embedded in the material can rust and discolour when the surface is painted. These problems can be treated by sealing the affected areas with good primer, one that resists alkaline.
Rust staining can occur where old nails are left in the substrate, or a wire brush has been used to prepare the surface. In this case, prime those areas with a good metal primer.
Steel reinforcing in concrete may be too close to the surface and cause rusting. The only effective way to resolve this problem is to get back to the metal itself and treat the cause of the rust then make any necessary repairs to the surface and repaint.