did you read the cans to see if the type of paint would cause a chemical reaction with another type of paint? they usually have some sort of warnings, like how enamel paints don't mix well with acrylic type paints. If you used the VHT wheel paint, you shouldn't have needed to use clear coat. If it wasn't due to mixing two different paint types then maybe it was because you didn't leave enough time for the paint to dry before applying the clear coat.

edit: seeing as how you used "enamel" clear, it was probably due to the different type of paint....