Any kind of abrasive blasting will ruin aluminum alternator housings. That's why rebuilt units can't be restored - the rebuilders blasted them.

Jim at Dixie has recently just started adding a chromate to the cast aluminum parts that duplicates the factory coating. This might be some kind of Alodine. I returned my alternator to him because I messed up and asked for the wrong pulley. He gave me the correct pulley, added the new coating, and re-tested it for no charge, just the freight. I suspect he had to disassemble and re-assemble to add the coating, but still no charge.