I haven't a clue on the grade of cast. I've encountered the same problem on automotive, motorcycle and sporting parts that were cast alum. The parts were beaded and polished, etched, DeOx/DeSmut. When the parts come out of the anod. tank, they are med. to dark gray and you can see what looks like something that was not mixed good in the metal. I am still trying different processes but haven't found anything to give the desired results.
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