I wasn't previously aware that the M103 cam cover is magnesium. Some years ago I researched magnesium finishing for a friend who got tired of polishing the Halibrand magnesum wheels on his Safir GT-40. Just siting in the garage the polished wheels would rapidly degrade to a dull, "fuzzy" looking finish due to oxidation and looked terrible.

I contacted friends still in the aerospace industry but came up blank. Helicopter gearbox casings are often magnesium, but apparently they are just left bare.

My M103 valve cover started to develop a few bubbles and now has more and a few breaks. The coating is very thick and I'm not sure it's paint, but maybe some kind of liquid plastic, and I think it serves to attenuate valve train noise in addition to improving cosmetics.

I know some vintage Porsche flat-sixes have magnesium cylinder case halves, but I'm not aware that they are anything other than bare. Of course you can't see much of a Porsche engine when you open the engine cover, but on a M103 Merc that big cam cover dominates the view.

If anyone knows of an "industry standard" way of finishing magnesium, if such exists, I'd sure like to know.

Duke