Why do plaster and paint blister above and below windows, and how is that repaired?

The areas right above a window lintelLintelThe beam over a door, window or opening in a wall, which carries the bricks above it and passes their weight to the sides., around the roller-shutter box and beneath the window sill near the radiator are often the first places inside a room to blister or show yellow damp stains. Rainwater finds tiny cracksCrackA 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. along the outside frame or under the marble sill and soaks into the wall.

Above this window, the 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. had blistered and flakedPeeling 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. away. All loose material was scraped back to a solid base, rebuilt and sandedSandingSmoothing a surface with sandpaper or a power sander to remove bumps, loose old paint or gloss, so the new paint or varnish grips well and comes out smooth. flush with the surrounding wall, and then painted.

Likewise, where damp stains have formed beneath windows and beside radiators, the damaged patches are scraped, filled, sealed with stain-blocking primerPrimerThe thin first coat that goes on before the main material. It binds dust, seals the pores and helps paint or waterproofing stick firmly. so discolouration cannot bleed through, and the room is repainted.