buy 5 different grade of sand paper, from medium to very very fine. use all of them step by step until you smooth out the surface and get rid of all the dirty thing.
use a buffing machine and a metal/rim rubbing compound to polish it until it shines. Then just use car wax to protect it so it will last longer.
well, i bet there's better way out there, but this one would work just fine if you know what you are doing.