Rustoleum 2k Ultra Cover Flat White Paint+Primer Cracking

Hello, I got some Rustoleum flat white for a model I was doing. I only needed a couple areas to be white but thought since it also said primer it wouldn't be too bad if I got it over other places I needed to spray over. I went over it with a rustoleum paint and it cracked very badly, but I chalked it up to the high heat that day (high 80s). I am using it for another model and wanted to check if it would work this time (high 70s and only 70% humidity) with a Krylon paint and that also cracked a little but not as bad. Anyone know what I am doing wrong or if it's just paint reactions? I want to paint the outside of this model white at least and then go over it with a gloss coat, I plan to test this before but should I also be worried about that cracking?