Did you use a primer? If you used a self etching primer you can't top coat some paints directly on the self etching primer. You have to coat with a sandable/fillable primer on top of the self etching prior to applying an enamel finish. I made that mistake. I painted bare metal with self etching primer and then applied Rust-Oleum automotive enamel. It looked great at first and then started to crinkle.
