Render (plaster)

Render is the layer of mortar that covers walls and ceilings, inside and out (indoors it is often just called plaster). It usually goes on in coats: a first, rough coat that grips the brick or concrete, then thinner coats that bring the surface straight. Finally the wall is either filled and painted or tiled.

Over the years, damp, saltsEfflorescence (salt deposits)A white, powdery or crystalline crust on the surface of a wall. It is salts carried by water through the masonry, left behind when the water evaporates. and small movements in the building make render blister, crackCrackA split or gap in render, concrete, a wall, a coating or wood. Fine cracks are usually only on the surface; ones that grow, keep reopening or run diagonally need attention. or go hollow, meaning it comes away from the wall behind while still looking as if it is in place. Hollow renderHollow (blistered) plasterPlaster that has come away from the wall or ceiling behind it. It sounds dull when you tap it lightly, and often blisters or cracks before it falls. sounds dull when you tap it lightly.

If the render underneath is not sound, no paint will last on it. So before painting, anything loose is removed back to a solid base and the gap is filled with new render. If the cause is damp, that has to be dealt with first, or the new render will fail the same way.