These appear as a result of salts crystallising in materials such as bricks and plaster or render. The fluffy deposits can be removed by dry brushing, repeated every few days until the deposits disappear. Harder deposits can be sanded to roughen the surface, then painted over. Conventional solvent-based paints should not be applied on new buildings where these can often occur, for at least 12 months, to allow the surface time to dry out thoroughly. Use a reputable primer prior to applying any solvent-based finishes.