Well, on my S500, the "protective coating" has hardened and flaked off in pieces... on top of that, its badly discolored to a golden brown color... it looks like it was originally sprayed on the front of the engine, covering the water pump, thermostat housing, and about the front half of the valve covers. I'd say about 50% has flaked off on its own over the years. The remainder is very hard and totally resistant to any solvent I've tried.. incluiding lacquer thinner, kerosine, etc. Looks like crap. "Less than attractive" doesn't do it justice. :-)
I'd like to remove this remaining "stuff", clean the aluminum of its corrosion before the already apparent pitting gets worse, and recoat it with a fresh coating.
It might come off "inadvertently" on newer engines, but can't imagine that happening on mine... unless I was using dynamite under the hood for some reason...

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KenP

1996 S500 W140 Coupe