Used jol's posted method above to clean up my $4.75 Ebay stem... well $13 after shipping.

This was the before pic from Ebay:



After:



There were some pretty good steer tube marks on the lower end which I sanded out as well as the scratches on the top of the stem. Used 320/400/600 for the scratches. I then used Easy-Off oven cleaner. I let it sit on the stem for 15 minutes and then rinsed. Used Mothers Mag & Aluminum Polish to clean it up nice and then used McGuire's swirl mark remover which took out the small scratches. I'll prolly polish it up a bit more. Amped how this turned out at any rate.

Also - if you have bolts with dirty/rusty heads, you can clean them up pretty easily with a drill. I put a few wraps of tape around the stem bolt threads and then inserted it into my power drill. I used 600 grit to clean up the head with the drill running.. If the head is rounded, place the sandpaper on a towel. The cushion of the towel will allow the sandpaper to better contour the surface of the bolt head. Holding the sandpaper can get a bit hot in your hand. Then put some polish on a rag and you can hold the rag or set it on your workbench and just push the bolt into it. Worked great.