Yes, migration of contaminants from the underside of fiberglass that lifts patient or creates pimples in the surface is a very common issue for old Corvettes and very difficult to remedy. That’s one of the reasons I used bed liner on my ERA instead of lacquer paint. I also rolled and brushed it on. Most old Corvettes have press molded, smooth on both sides panels (except for earliest years) and have a more tightly sealed underside surface than most hand laid or chopper gun fiberglass which is fairly rough and porous by comparison. So if battery acid, brake fluid, accessory
oil can get through a Vette panel it potentially could do so even easier on a hand laid body. Paint alone may not seal the underside glass without applying a lot of it.
Cobra fiberglass bodies do have one advantage over old Corvette bodies which is the gel coat surface which should keep most contaminates from migrating to the surface. Old Vettes never, ever had gel coat surfaces. It was not practical for early, mass produced, match mold, steam cured fiberglass panels.