It depends how far you wanna go and how much you wanna spend. It's alot easier to paint rims with tyres unmounted.

I bought a cheap sandblaster attachment for my air compressor years ago. You can blast all the old paint/rust/dirt of pretty well. Then just spray your chosen undercoat/base coat/clear coat.

If you want to do it without taking the tyres of the rims you can smear the tyres with old axle grease or oil first. then the overspray just wipes off and you get a tyre shine for free <img src="/forums/images/graemlins/lol.gif" alt="" />