[BoxsterLS376]

Yeah I think the wheels needed more prep work before moving forward with the coating. Hard to say without seeing them first hand but based on the pre-coated pictures I can see a lot of scuffs and bumps which all contribute to the finished product, that and a proper cleaning like jcslocum said.
I wouldn't give up just yet, practice on something smaller first. You should see how crappy my first few attempts at fiberglass have been, UGH!
$.02
J