Efflorescence (salt deposits)

Bricks, concrete, plasterRender (plaster)A layer of mortar (sand, cement or lime, and water) spread over brick or concrete to give a wall a straight, sound surface ready for paint or tiles. and the ground itself contain soluble salts. When water passes through a wall, it dissolves them and carries them towards the surface. There the water evaporates and the salts stay behind, as a white powder, a fluff or a hard crust.

It often appears low on walls with rising dampRising dampMoisture from the ground that climbs up inside walls through the pores of the materials. It shows as stains, blown render and salts at the foot of walls., on walls below roofs and balconies that no longer keep water out, and in basements. When the salts crystallise behind the paint or inside the plaster, they push it outwards, and the paint bubbles and peelsPeeling paintWhen paint loses its grip and lifts off in flakes or sheets. It is almost always a sign of a problem underneath, usually damp or poor preparation..

Brushing removes them from the surface, but if water keeps passing through, they will come back. Before painting, the damaged plaster is removed and the cause of the damp is found. Paint on its own won't stop them.