I use WD40 and 0000 steel wool on dirty aluminum cases. Four-aught wool is fine enough that no scratches will show. The WD40 is a solvent that is mild, but complements the steel wool well. In addition, the WD40 leaves a film that protects the aluminum and makes future cleanups easier.
If that doesn't work, soda-blasting will do the job. Baking soda is a great media for aluminum as it is mild-yet-effective and the residue will dissolve with a little water.