Not all cracks are the same
A fine web of lines across a door, a single crack running up a wall from a window corner and a gap opening along the top of the skirting all look like “cracked paint”, but they have different causes. Some start in the paint film itself. Others come from the surface underneath moving, shrinking or settling, with the paint simply showing what is happening below. Painting straight over either kind usually means the cracks return, so the first step is working out which one you are looking at.
Crazing: fine surface lines
Crazing is a network of small, shallow cracks in the top layer of paint, a bit like the pattern on old china. It often appears on gloss and other hard finishes as they age and lose flexibility. It can also happen when a coat goes on over one that has not fully dried, or when a hard paint is applied over a softer one, so the layers expand and contract at different rates. Light crazing can usually be sanded smooth and repainted once the cause is understood.
Alligatoring: deep, scaly cracking
Alligatoring takes its name from the pattern of large, squared-off scales it leaves, like reptile skin. The cracks are deeper and often go through several layers of paint. It is commonly found on old woodwork that has been repainted many times, especially where oil-based paint has gone over incompatible coatings or where a glossy surface was not keyed before the next coat. Because the whole build-up has failed, it normally has to be stripped or scraped right back to a sound base before repainting. Sanding the top alone will not stop it spreading.
Thick coats and mud cracking
Paint put on too heavily in one go, or loaded into corners and around mouldings, can dry with cracks that look like a dry riverbed. The surface skins over while the paint beneath is still wet, and it splits as it shrinks. This is common with emulsion rolled on thickly over porous patches, or with filler-heavy textured paints. The fix is to scrape or sand back the cracked area, prime where needed, and build the finish up in thinner, even coats, letting each one dry.
Hairline cracks in plaster
Thin cracks in walls and ceilings are very often the plaster rather than the paint. New houses, extensions and freshly skimmed rooms commonly show fine cracks as the materials dry and the building settles. Older homes can crack where ceilings meet walls, along plasterboard joints, and around door and window openings as the structure moves slightly with seasons and temperature. These are usually cosmetic, but if a crack is wide, keeps growing, runs diagonally across brickwork or comes with doors starting to stick, it is worth getting a builder or surveyor to look at it before redecorating.
Flexible fillers and caulk
Ordinary rigid filler is good for holes and stable cracks, but where two materials meet and move, it tends to crack out again. That is why decorators use flexible fillers and decorators’ caulk along the joints between walls and ceilings, around skirting, architraves and window boards, and along the joins in plasterboard. Caulk stays slightly elastic after it has cured, so it can take small movements without splitting. It needs to be paintable, applied neatly and given time to set before painting. For larger cracks, raking out loose material and using a scrim tape or a flexible filler gives a longer-lasting repair.
Timber moves outside
Exterior timber, such as window frames, sills, fascias, doors and cladding, swells and shrinks with rain, sun and frost. Rigid or brittle paint cannot keep up, so it cracks, especially along joints and end grain. Water then gets in through the cracks and the paint begins to lift. A flexible exterior system designed for wood moves with the timber far better. Preparation matters just as much: cracked paint is scraped back, the timber is allowed to dry, bare areas are primed and joints are sealed. Soft or rotten timber should be repaired by a joiner before any paint goes on.
The proper repair
A lasting repair follows the same order whatever the cause. Remove loose and cracked paint back to a firm edge. Sand the area so there is no visible step where the old paint ends. Fill holes and stable cracks with a suitable filler, and use caulk or a flexible filler on moving joints. Prime any bare plaster, timber or filler. Then apply the finish in the right number of even coats, following the drying and recoat times on the tin. Skipping any of these steps is the most common reason cracks reappear within months.
When to call a decorator
If the cracking covers a large area, keeps coming back, or affects old woodwork with many layers of paint, a decorator can work out the cause and the right repair. Our interior painting and woodwork pages explain how we prepare walls, ceilings and joinery.