If the aluminum is heavily oxidized, you can start by cleaning it with Barkeeper's Friend, which is a very mild abrasive. You can quickly and easily bring the metal to a satiny sheen with that and then start polishing with Mothers until you turn the sheen to a shine. As others have stated, a buffing wheel on a drill will work wonders.
The same procedure also works on Electrolux PN2 and PN4 upper shells with a polished aluminum finish. I actually tried stripping a painted PN4-A shell last year with the idea of polishing it up, but I ended up giving up and painting it instead. It ended up coming out pretty nice in the end.