I agree that using a base/clear combo will not be a problem, UNLESS the problem starts at, or under the old paint.

I always discourage using primer as a final finish. You are painting it to look better, but in fact it will fail pretty quickly, and then, guess what?....It won't look better, and then you have lost both your time and your materials cost. Why not just do it right?

I don't take any chances. Whenever I expose bare metal I ALWAYS metal prep it...PERIOD! The rusting can start simply from the moisture in the air, and it's microscopic, so you can't see it....and you know what? I NEVER have rusting under the paint, since I started this practice over 20 years ago. I've read many people's advise that you should sand, and then prime immediately, but I like to sleep at night, so I just do it. I mean it takes a few minutes, and you can do a whole car for $5. Why would a painter even consider risking the loss the entire job for that? It is by doing things this way, that I can guarantee my work as long as a customer owns it! Beside that, the only cure for rusting is to strip and start over, and also, I hate painting anything twice! :-)~