Originally Posted by Maniacmechanic1
I have heard of stress cracks in C3 Glass. Never witnessed till now.
I agree with 7TRoadster and Maniacmechanic1. The depth of the crack is anybody's guess at this point. Could be in the fiberglass itself or maybe on the surface. You didn't mention if the defects show up in the exact same place each time, but I'm assuming that's the case.
Just my opinion, but in my experience if a defect shows up that quickly then it's almost certain to be some kind of reaction between the paint and some unknown chemical residing in what is most likely a surface crack or scratch. Remember that primer can be slightly porous, that's what gives the paint something to bite into.
I have done jobs on metal body cars and on a couple of them I've had defects quickly materialize that look remarkably like your picture. Both cases turned out to be incompatibility between the paint that I sprayed over existing topcoat. It looked exactly like a crack, but had to be a virtually invisible scratch in the old topcoat or primer.
Back when the Beatles were popular and I had hair, I quickly learned that refinishing using lacquer over the then-common acrylic enamels was a good way to use up a lot of sandpaper. Everywhere there was a sand scratch the lacquer chemicals dug in and caused either a ridge or canyon to develop. End result looked exactly like a crack.
It would help us to know if you are going over original factory paint or a repaint from years past, what kind it was, what you are now using for primer/sealer, final finish, prep technique, etc. One thing is for sure, don't keep doing the same thing while expecting different results. Something is clearly wrong and you need to find out what it is.
To be sure you should probably remove all previous paint, right down to the body. Don't sand it like you would a metal panel. It can be done, but one slip-up will create a bunch of work to fix. I'm currently using the stripper method recommended in 7TRoadster's sticky. A little slow but you will wind up with a body free of sanding induced damage.
Good luck, don't hesitate to ask questions here!