Oh and BTW ... ALWAYS wear good strong gloves! I use leather welding gloves and a full face mask, at speed a grinder runs with a buffing wheel one slip and you WILL be missing skin and it is VERY painful. The gloves will also protect your hands from the heat generated on the aluminum it can sometime get hot enough to burn you. Also wear you worst clothes as they will be black after you are done.
I am slow doing this so don't expect a lot of progress, just wanted you to see that polishing is easier then it looks and you don't need anything more the 400grit sanding. Let your buffing wheel and emery compound do the hard work.
All I use is my makita grinder with grinding disk to remove casting marks and casting joints, then a 5" orbital sander with 220 discs, then put a buffing pad on my makita grinder and it will easily remove 220 grit sand marks.