Painting with Rustoleum spray paint

I use Rustoleum spray paint a lot. In some cases I need to I spray Rustoleum clear coat as a finish coat. The results are not always good. I will have a nice smooth glossy finish and when I spray the clear coat the paint almost immediately starts to "crack". This occurs more than 50% of the time. I have sprayed very thin first coats of clear and that does not seem to work. Any suggestions.