[WarrenG]

The book I have recommends an acrylic or vinyl acrylic coating baked on for maximum adhesion for aluminum and magnesium. It states that most instances of oxidation after coating have been due to failure of the coating (sometimes pinholes not readily visible to the eye) that allow oxidation of the metal beneath the coating. If the magnesium is changing color due to brake heat, I'm not sure what (if anything other than cooling air) would stop that.