Ermm, fine grit...
I have aluminium oxide sandpaper that gets the metal pretty smooth, then something like 400, and finally 800 grit should do the trick, before using 000grit steel wool and the mothers polish. I havent actually practiced getting a really good finish, but I have been reading up on it & asking questions to those who do- Candyman (search for 'maxxbling') does beautiful things with aluminium, leanrt a fair bit. A polishing wheel on the dremel with the mothers would be great too for all the tight spots and curves.