I recently painted a 75 Vette. When I got the car it had several places on the hood that the paint was blistering (not just the clear delaminating). It seemed to be coming off directly from the gelcoat. I sanded, primed and subsequently applied a water based primer but the same problem reoccurred. I tried re-sanding and leaving outside in the heat for a couple of weeks at the suggestion of a colleague but once again the blistering occurred. Thankfully this is an extremely understanding customer and he is not giving me a hard time. Does anyone have any suggestions. Is there a blocking agent I can try? Has anyone ever experienced this before. Any suggestions?

Sounds like the gel coat (or just underneath it) is contaminated with some left over solvents. Has the car ever been chemically stripped? If that gel coat was stripped hard and softened it might have had some breakthroughs where stripper residue made it down into the fiberglass core. Stripper residues consist of the active M.C. solvents, paraffin wax, and metho-cell thickener (think corn starch). It's like a self-sealing liquid compound that becomes active and gasses every time more solvents get over top of it. Someone on here had also mentioned that aggressive soda blasting had trapped some residue down in the fiberglass and they were getting some bubbling like this as well......

That sounds like it could be the problem. Do you know what can be done at this point. Is there something I can apply that will block the solvents from interacting from subsequent top coats.

Well, you've already tried my "go to" solution of using the sun to bake out the problem. Next thing for me is a little more radical.... I would be opening up the surface area with a moto-tool, use a heat gun on it, let it sit in the sun again, and "try" a barrier over it of some finely chopped up mat and resin. Back out into the sun, let it set, and check for problems. Finish off with your favorite bondo or poly icing.

Oh, and I don't want to get you or me in trouble here.... If that looks like a "factory" gel coat.... well, they didn't exist. It would just be the layer of first resin that was part of the press mold or it could also be a separate surfacer that was used on the later SMC style parts. Not sure on when the exact crossover was yearwise in the tech. Corvette guys can fill books about this stuff.