You need to make sure that the room you are painting it in is VERY hot, like 40-50 degrees. And yeah, you ll need to sand it heaps, youll probably get away without getting rid of the top coat. It will be a pain in the ass, can gurantee that.
While you are at it grab yourself some putty and fill little scratches and pin dents as well. If you can heat up the room try to get hold of one of those heat lamps panel beaters use, the results will be much better.
If you want metallic-ish paint you ll definately need clear, and a thick coat of it on top, that is what gives it depth.
Igor.