Roger, Polyfiber sells magnedyne by the quart, which is cheap. You
prep by rubbing any powdery corrosion away with scotchbrite, then with
an alkaline solution (caustic soda), then application of the
magnedyne, which is a dilute chromic acid. Directions are on the
label.

Careful, because the chromium is hexavalent. MSDS can be found on the
web.

There are other treatments, and I suspect the alodine chromic acid
treatment for aluminum will work, except you don't use phosphoric acid
as a pre-prep.

I have seen other details on using irridite or something of the sort
you mix up in AC 43.13 and also in a Continental engine rebuild manual.
Many of the Continentals have accessory cases cast from magnesium.
It is lighter than aluminum and usually of a darker color. The last
magnesium I did was with Polyfiber's treatment and it made the metal
appear iridescent and darkened it a little. Then I painted it --- a
rear case --- with Continental gold. Whereupon it was just
scintillating. It rocked!