Blisters appear as swellings on the surface of the paint and vary considerably in both size and density. Coarse blisters, larger than 1.5 mm in diameter, generally occur in patches, although they may also be found in isolation. Fine blisters, typically from 0.5 mm rings, meandering lines or in the shape of a finger print. Blisters in the colour coat are generally more prominent than those occurring between paint and sub-strate.
Prepare substrate correctly – remove all corrosion and contamination.
If a primer has been left in humid conditions, ensure that it is fully dried out before topcoating.
Sand down to substrate and re-paint.