I hate to disagree with anyone, but I do believe in the "test of time".

I've been using urethane primer, and then Fiberglass-Evercoat Euroglaze over 36-40 grit rough prep for over 15 years with no problems. It's been one of those products that I never have to worry about. I do seal before painting, to prevent the porous area from absorbing solvents. That could also cause a shinkage problem.