I'm no expert and my bodyshop did the work, however...

The Porsche rims seem to have a clear-coat on top of polished alloy. I don't know the brand of paint.

I think you'll know when you've reached the metal. The grey colour you have encountered sounds like the primer. This can often be seen on the back or inside of a new wheel.

I think the polishing is achieved by spinning the wheel at high speed against a diamond or something.

Good luck!

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190