Here is how ive polished my old L series rocker cover and all my alloy wheels(including the watanabes so check my galley for results).

Degrease the alloy
If its pretty ratty, use 800 wet and dry sandpaper(plenty of water)
Then 1200 wet and dry sandpaper (or if its not so bad jump straight to 1200grit). still lots of water.
If you wanna go crazy use 2000 after... but i never have.
Then get yourself a air compressor and a 'whizzy gun' or 'gasket scraper' or something like that.Something that spins fast and you can get a nice small felt pad for.
Apply autosol/other good metal polisher(dont need to use heaps) and spread over all of the alloy.
Attack it with the felt pad on the whizzy gun at full RPM.

Brings it up beautifully! Doesnt take long either!