Why Paint Peels And Blisters

Paint that lifts has not gone off. Something under the film has broken the bond, and unless that is dealt with the new coat fails in the same place.

A paint film is a barrier under stress

Dried paint is a continuous polymer skin bonded to the surface beneath it. It is thin, slightly flexible, and holds on by a combination of chemical attraction and mechanical key.

That bond is under constant load from temperature change, from the substrate moving, and from anything trying to pass through the film in either direction.

Failure begins wherever the bond is weakest, then spreads, because a lifted edge lets more moisture underneath and the damage propagates outward.

Moisture from behind is the main cause

Water vapour moves through walls from the warm side to the cold side. If it reaches a relatively impermeable paint film, it condenses under it and forces the film off.

This is why blistering appears on bathroom ceilings, on external walls with a damp problem, and on timber where water has entered through an unsealed end grain.

The distinguishing sign is that the paint comes away with the surface still sound underneath. The film was pushed off rather than pulled.

Contamination and surface condition

Grease, dust, chalky old distemper and silicone residues all stop paint wetting the surface properly, giving a bond that looks fine until it is stressed.

Glossy surfaces fail for a related reason. There is nothing for the film to key into, which is why abrading and priming exist as separate steps rather than one.

New plaster and bare timber absorb the binder out of a topcoat unevenly, leaving a weak, patchy film. A mist coat or a primer solves it by sealing the porosity first.

Incompatible layers

Putting a hard, inflexible coating over a soft one means the top layer cannot follow the movement beneath it, and it cracks and then flakes.

Old houses often carry many layers accumulated over decades. The failure plane is frequently deep in that stack rather than between the last two coats.

Where the layers are unknown, a small test patch prepared and coated in advance tells you more than any assumption about what the previous owner used.

What repair actually requires

All loose material must come off, back to a firmly adhered edge, because painting over a lifted edge simply seals the problem in.

The moisture source has to be found and stopped first. Repainting a damp wall is a cosmetic delay measured in months rather than a repair.

Filling and feathering the edges of a scraped area avoids a visible step, and priming the exposed substrate restores an even absorbency before the topcoat goes on.