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Ever notice that old Mahles get a dull, grey, rough, patina, and this would not occur if they were painted.
Umm... I've owned a few cars with dull, rough, patinaed paint. Paint can fade.
There definitely is a finish on it, because the factory wheels can be "stripped" with oven cleaner. Maybe something equivalent to an annodizing.
Whether it is a paint, powder coat, or some other type of "finish" is a different story.
My wheels were "oxidized" too, but as I stripped them (one mechanically) you could see the wire weel abrading through the silver, then a metallic blue film, then to the bare metal.
My advice... strip the wheels, do several light coats with Wurth Silver wheel paint, knocking down in between with 0000 steel wool, and then several light coats of Wurth Clear coat.
Or have them powder coated.
Here are my old wheels on my old 914. (Wurth method)