Polished mine last winter and here's what I did: Got the paint and scratches off with 120 grit attached to an air tool, Then hand sanded it with a rubber sanding block starting with 100, then 120, 150, 180, 220, 280, 320, 360, 400, 600, 800 and 1000 with plenty of water (I heard WD-40 works as well), then used polish to shine it. It was just like a mirror! You could skip a few grits, but it is actually going to take you longer to get rid of the scratches left by the previous grit. Believe me, I tried to save some time polishing the big aluminium molds we have where I work by skipping grits, and ended up having to work twice as hard...
It took me about 2½ hours to do my swing arm, but I love the result!:)