Why does paint stain and discolour on masonry?

There are a number of principal causes:

1. 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 Dulux Trade Alkali Resisting Primer when the surface is completely dry.
2. 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 locally with Dulux Trade Metal Primer prior to painting.
3. 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. Before repairing the surface and repainting.