The short answer is that some substance, whether it was moisture surfacing from underneath, a solvent, or a contaminant trapped under the surface of the paint, when it cured and formed a skin, it blistered up from the substrate in that spot.
There are a few main things that can contribute to getting blisters:
- Too much paint was applied, and when the paint skinned over on the surface, there were solvents trapped underneath.
- Paint was applied in direct sunlight: Direct sunlight will help accelerate the cure rate of paints, regardless of color or ambient temperature. If this acceleration happens too quickly, the solvents used to thin down the paint are not able to evaporate and escape fast enough.
- The underlying coat of paint or primer was not ready to be overheated: Many paints and primers will recommend an absolute minimum overcoat window. This information should be used with caution, because the paint may still be curing or have solvent evaporation taking place. If the underlying coat is still emitting solvent vapors, blisters can form in the subsequent coat.
- Surface contamination can 'reject' areas of paint, even if it appears that the paint has cured to a film. A blister forms where the paint was rejected from the rest of the substrate. The 'contaminant' can be any substance that prevents the coating from adhering, curing, or functioning as it is supposed to.