Rouge. Even the kind used for facial makeup. It's a VERY FINE abrasive, used more for polishing than removing scratches, but with enough work it'll get really shallow scratches out. If you use something coarser for a deeper scratch and it dulls the aluminum, rouge will polish it back up. If you're doing it by hand, it'll take a LOT of gentle rubbing. If you use a cloth wheel on a Dremel-type tool, practice on scrap first (like the bottom of a can) so you get a feel for how much pressure will polish and how much will eat through the finish.
(The first time or two won't be "easy", but once you get the hang of it, you can almost do it in your sleep.)