Interested in this one too, I'm planning on doing it to my timing cover.
Ive seen people on the VW forums polish cast aluminum by sanding it starting with a medium paper and gradually getting finer untill they are at 2500 grit. Using wd-40 as a lubricant on the paper is supposed to speed up the process and help the sandpaper last longer. Once you get to the finest paper you can find, then you buff it to a shine.
I started doing this to my shift socket, and got it nearly to a mirror shine with sandpaper but then I gave up because they are just pot metal and you can't really get them completely smooth because they will just start pitting.
The results look pretty good on aluminum but I'd be worried about it oxidizing so you'd probably want to apply a high-temp clearcoat to it or something.
The aluminum on the timing covers is pretty smooth from what I have seen, so I'm going to start with a 1500 grit and see how that does. If that scratches it more than it polishes I will go up to 2k or 2500.
I think polishing is probably the way to go as its pretty hard to get paint to stick to aluminum in my experience.
I am tempted to have the thing
chromed... along with the shift lever (why Ford chose not to chrome the shift handles is a mystery to me :roll: ) and some other pieces.