1957 Chevy Stepside Pickup

Could it not be a chemical reaction between the type of filler used and the paint?. As a rule of thumb I normaly give the kit a coat of Barcote Isolating primer before the top coat as it dries a sandy colour and picks out most of the imperfections plus dries in 5-10 mins . With mild heat. If you have ever rubbed a 1.1 car down in factory paint you may well have came across it. As they usualy get Isolater then Primer then Topcoat . The stuff I use is premixed so you have no faffing about thinning it down. You can even put auto paint on top of enamel with the stuff as a barrier.