Acid, if there not polished, then two step them. Spray them down with an acid, come back behind that and hit it with soap. then rinse. Repeat if necessary. They should clean up very nice. Don't let the acid sit to long.
Just like like Ron said...Also if your step 2 is caustic base (sodium hydroxide) don't mix it too strong or let it sit on too long as it will burn the aluminum leaving brown streaks.