Description of problem:
Even the best paints will let water vapors through. When the vapors go inside the film, they can lead to the weakening of the adhesion between different coats. Bubbles containing water may show through.
Causes:
Contamination of the surface before spraying.
Insufficient ground coat.
Insufficient topcoat.
Inappropriate thinner.
Final coat was exposed to high humidity before it was completely hard.
Continuous exposure to high humidity / contaminated air supply.
Residue of sanding water in corners, edges, rebates and below decorative strips.
Insufficient isolation of polyester.
Remedy:
Sand carefully until all the bubbles have disappeared: In other words, if the bubbles are in the topcoat, it will have to be removed. There is always the possibility that the sanding residues fill the holes of the bubbles. The humidity that is withheld may damage the substrate. The only way to get around this kind of problem is to sand all the way through to the bare metal, clean with Valox, sand and repaint.
Note:
The presence of these types of bubbles around the headlights or taillights may be an indication of rust coming form underneath. Some micro holes in the metal might be letting the water through. The bottom of the vehicle should be well protected.