OP,
A number of things come to mind when looking at the paint on you frame.
Either the paints that you were using were not compatible with one another, even if they were from the same manufacturer. etc.
Another reason may have to do with the surface preparation itself any contaminants can cause this to happen BUT I suspect that the cotes before the clear was applied may have caused the crazing causing the topcoat to crack and craze that along with storage temps. etc.
I have used rattle cans in the past and they
all do not have these issues...certainly not as strong as a baked on factory finish, but if a catalyst top coat is used they can and do provide good results....Been there done that.
Personally I like the "patina", it gives it a bit of character.
Best, Ben