If there is blemishes in the alum itself you could sand them up in stages.If you are talking just grime, a good polishing should do the job.You could if you wanted just wet sand them with like a 1500 it will have them looking shiny with out damaging the surface before you even polish.I have always hand polished all my motorcycle parts,car wheels are big so you might want to try out that powerball mentioned above,ive heard great things about it and you hands will thank you..