I have done the following to get a close to factory look on an aluminum frame.

I first clean it up really good with something like Mothers Mag Polish (yes it will polish it), then i take something like a 00 scotch brite and "scuff" it up in the same direction (so I do long strokes down the entire lenght of the frame rail).

For the cast portion, I first scrub it really good with mineral spirits, blow it off with compressed air, then use contact cleaner to realy clean up the cast areas.

Works pretty well.
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Charles Gallant