That's the pits mate, but what was the common denominator in the 2 paint jobs? I layer my automotive clear on pretty thick, including having it go milky, and usually only 5 mins after the base coat has dried, and NEVER had it crack
on me like that, the only time i had an issue, was a bad can of primer. If it goes milky i hold it under the spot light till it clears up, then throw more paint at it again, this method works
on new kits and ones that have been painted a few times before. The only other think i can think of is mixing brands? I only use automotive primer, automotive acrylic base colors, and acrylic clear and pearls aswell.
GOOD LUCK next time around mate.