CheeseButtons is right on with the fine grit paper and buffing compound...
here are some tips taken from
www.streetrodstuff.com
Buff large aluminum parts, such as a beer keg like mine, without having to use a fixed buffing machine. Take your 7-inch sander/grinder/polisher and replace the pad and rubber backing disc with a large cotton buffing pad. Be sure to use large fender washers on each side of the pad for stability. Start sanding the aluminum with about 600 wet/dry using soapy water. Continue with 1000 grit until everything is smooth. Then hit it with the buffing pad and Red then White compounds. It takes awhile so don't get discouraged. Once satisfied, switch back to the felt polishing pad and use 3M polishing compound (or similair) and buff as you would paint. Works great and saved me $160. That's StreetRodding Stuff.
you could probably use a bench mounted buffer instead of a hand held one, it will probably give you more control sonce valve covers arent that huge.