Purple power is a strong degreaser that has "brightner" in it and i agree with Mike, its a great way to cut the lardy caked on sludge accumulate. You may have to agitate with a stiff brush after it has sat for a few minutes. Then, find a truck wash( big rigs) or similar place that will sell you straight wash acid. It will disintegrate any remaining grease or residue and leave a nice etched surface suitable for painting or polishing. Do not leave acidic products on aluminum for too long, they will start to dissolve the metal.