Why is my new paint cracking and crazing?

Paint cracks when the film is put under more tension than it can take, and where that tension comes from is what identifies the fault. The pattern is genuinely diagnostic: the scale and the shape of the cracking tell you whether the problem is in the last coat, in the layers below, or in the wall itself.

What you'll need

• a scraper and abrasive paper
• primer or a suitable sealer
• a torch for raking light
• filler, if the substrate is cracked (optional)

Step by step

1. Look at the scale and the pattern.Very fine random hairlines across the film, a coarse scaly pattern with visible depth, or discrete cracks running in lines. Three faults, three causes.
2. Fine crazing across the film means the paint dried too fast or too thick.The surface skins before the body below it, and the shrinking surface tears. Heat, sun, draught, or a heavy coat laid on all do it, as does a hard brittle coat over a more flexible one.
3. Coarse alligatoring means the layers below are the problem.A film applied over an incompatible or insufficiently cured undercoat, or many layers of old paint that have become brittle. The whole stack moves as one and cracks in large islands.
4. Cracks in lines mean the substrate.Follow one with a fingertip. If it runs along a plasterboard joint, a plaster crack or a filled repair, the paint is reporting movement rather than failing.
5. Test how deep the cracking goes.Scrape a small area. If a blade lifts the whole paint stack off in flakes, the failure is deep and the surface has to come back to something sound.
6. For film-level crazing, sand back and recoat properly.Abrade to a sound smooth surface, prime if bare patches appear, and apply thin even coats in moderate conditions — not in direct sun, not with a heater on the wall, and not one heavy coat instead of two thin ones.
7. For alligatoring, strip back.There is no coating fix for incompatible or brittle layers. Take the paint off to a sound base, prime appropriately for what is left, and rebuild.
8. For substrate cracks, repair the wall.Rake out, tape if it is a joint or a recurring crack, fill in thin coats, feather wide, prime the repair, then paint. Painting over the crack reproduces it.
9. Check the paint compatibility before recoating.A hard gloss over a soft or unsound layer, or a water-based coat over an unprepared oil-based one, sets up the same tension again.
10. Control the conditions while it dries.Moderate temperature, no direct heat at the surface, ventilation without a draught blowing across the wet paint.

Tips

• Old paint in a pre-1980 house may contain lead. Do not dry-sand or heat-strip it — get it tested and use wet methods or a specialist if it has to come off.
• Crazing that only appears near a radiator, a window in full sun or a heater tells you the conditions were the cause, and where.
• Two thin coats almost always outperform one thick one, in appearance and in durability.
• If a wall keeps cracking in the same place after several repairs, the problem is movement and needs diagnosing rather than repainting.

Common mistakes

• Applying one heavy coat to save time — The surface skins and the body underneath continues to shrink as its solvent or water leaves. The skin is put in tension by a still-moving layer below it and it tears into a fine web of cracks. Thin coats dry through evenly.
• Painting a hard finish over a soft or unsound one — The hard film cannot follow the movement of the layer beneath it, so it cracks and eventually flakes. Whatever is underneath has to be sound and compatible, or it has to come off.
• Painting over a crack in the plaster and hoping — Paint has no ability to bridge a moving crack. It reappears, usually within a season, and now there is a paint edge along it as well as a crack.

If it doesn't work

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