As far as polishing aluminum goes, sorry for the photos of my motorcycle, but this is how I did it cheaply. In the case of the Crack panel, you can probably start at 800, and work your way to 2000 or 3000 grit sandpaper. At that point you need some sort of high speed buffing technique using buffing compounds of finer and finer grit. It can be done cheaply with an electric drill using cloth buffing wheels, but the higher speed you attain, the more mirror-like your results will be.
This is what 1000 grit sandpaper does to aluminum
It doesn't look much different from what ships from the Bottlehead company in terms of the way it catches and reflects light.
I used cheap Harbor Freight buffing wheels that cost about 8 dollars for a three pack. They look like this:
http://www.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/i/m/image_17472.jpg
Then you buy an assortment of buffing compound sticks that look like this.
http://atomictoasters.com/wp-content/gallery/sharpening101-7/buffing-compounds-4oz-bars.jpg
You use grittier or finer compounds on your three wheels. And don't ever mix them up. That way you get a more mirror-like finish with each finer buffing wheel you use.
This is what my engine looked like after getting after it with the drill and buffing compounds.
Sorry for the off topic photos, but I wanted to let you know that no sand paper, even 3000 grit auto body stuff will give you a mirror finish. It'll look like 'brushed nickel.' You need to use some sort of higher speed buffer with compounds to pull out a reflective finish.