I'd highly advise against powder coating anything ferrous (with iron in it), as powder coating only lightly bonds to the surface and its the rigidity of the paint that keeps it in place. ANY moisture getting in anywhere and rust will spread like wildfire underneath and you'll never see it. Black hammerite rattle can is the way to do it, get a wire brush attachment for an electric drill to remove rust, use degreaser and then washing up liquid to clean and then just use 400-600 grit sandpaper to rough up (or 'key') the surface. You can stick playing cards between the rim and tire all the way around rather than mask off with tape if you're not taking off the tires. Gloss black will probably be slightly harder wearing but I've got no evidence to support this claim.