If you want to paint your rims, you have a couple of choices. Im not quite sure what you mean by 'can I scratch up the existing paint'. Because if you're rims are polished/gunmetal color, there is no paint, just metal, and maybe a clearcoat. So if your rims don't have a clearcoat on them, yes you will need either a direct to metal primer, or a regular primer with an self etching formula, or an adhesion promoter before you put primer done, ONE of the three will work, if you do two or all three at a time, it'll cause problems. After you mask off the lip and spray the primer. If you have dirt build up or drips or sags, you can sand with 1000 or 1500 grit paper to flatten it out. Let it dry and spray the paint ,Then spray the clear coat. This is if you're using actual auto paint. Which you can do,
www.Alsacorp.com makes most of their auto paint in spray cans. If you're using Home Depot bought rattle style, then you'll probably end up getting a glossy paint. I recommed staying away from that and go with the good stuff. Most black rims out there come with paint from the manufacturer. I had a set of 18's which I rode on for a long time without any problems with chipping or cracking. The only other choice (besides buying new rims) would be to get them powdercoated, but then if you choose to do that, you can't keep the lip, because powdercoating is difficult to get edges like that. Good luck with it