scrub them,

sand then lightly with ~1000 grit

prime them (2 cans) with light coats (use duplicolor tire primer)

paint them (4 cans) with light coats until coated evenly and fully (again, duplicolor rim paint)

clear coat (2 cans) again, light coats.

there you go.

OR,

take the easy route - powdercoat them.

powdercoating is the only way to go...lasts longer than spray painting and heaps easier as DHV70 suggested