Causes

1. Solvent being trapped within primer or topcoat during the drying process
2. Insufficient flash-off between coats. Where a second/third/fourth coat is put on before the coat underneath has had enough chance to dry. The solvents end up ‘trapped’ between the coats and blister out over time.
3. Insufficient flash-off before force drying / stoving. This causes the solvent to literally boil and vaporise out of the film leaving hollow blisters in the paint film.

Prevention

1. Do not under-thin materials, the product can be too ‘thick’ and can lead to solvents being trapped in the film.
2. Allow primers and topcoats to flash-off (refer to technical data) before overcoating or force drying
3. Do not use very fast thinners in warm conditions. This often leads to a heavy application technique, where the applicator is trying to keep a level of flow but fighting the fast drying solvent. Using a slow thinner is advised and will often cure a blistering issue.

Rectification

The blistering should be flatted down to a sound surface and re-painted. Any attempt to simply paint over solvent blistering can often result in more blistering at a later date.