White powdery salt deposits can appear on any surface when contact with water occurs. It can often be seen that on external brick which appear clean an free salts. But during water weather the salt are activated and become apparent in a white format.
A paint coating and on the surfaces of new cement, plaster and brickwork. On redecoration projects where the existing building surfaces are painted and no defect is visible on the surface this type of problems can also happen. This can be due to the existing coatings being porous in nature whereby allowing moisture vaper to pass through the paint film. Any surface salts present and exposed could crystallise and be more difficult to remove if not attended to straight away.