Don't use sandpaper! Maybe try Mother's Mag and Aluminum polish. Although not sure if it will cut through the clearcoat on those or not. The "rust" as you call it (fyi- alum. doesn't rust, lol...) is most likely brake dust and road grime. I've used Simple Green and also LA's Totally Awesome cleaner before to remove tough junk. The LA's can actually be found at the Dollar General, but works great. Used it on my engine bay as well.