The spoiler on my car....
If it's something you bolted on because it's teh kewl, go ahead and give bicycleboy's advice a shot. A bolt-on spoiler would have already ruined any car, and if that's the case, even that bicycle advice would be an improvement.
Say, are they still painting vintage bikes with catalyzed polyurethane? That'd be great if they were, so we'd know that rattle-can enamel would definitely work, instead of guesstimating.....what kinda primer do you use to make those two dissimilar products work well together?
USS, you've moved on to another car by now, right? Or is all this jibber-jabber helping you decide?