I spray gunk engine degreaser (foaming kerosene) or whatever degreaser I have on hand. After it soaks for a while I blast it with the pressure washer on a low-medium setting. Works great. I've never had a problem.

I have had bad experience with Simple Green etching alloys - even diluted. I don't use it on stuff I care about. It's great for cleaning oil off of the garage floor and basic cleanup.

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D.H.

04 ML500
02 E430

Experience is something you don't get until just after you need it.