Clear coat cracks

I have a pair of e39 bmw headlights I’m restoring I’m using a wet sand with 600, 1000, and 2000 grit. Once I go to apply the clear coat I’m using a rustolium gloss clear but it starts to crack when drying and I’m not sure how to fix this I applied alcohol before and let it dry to make sure the surface was clean. What do I do?