The result I got would be hard to describe. I immediately erase images that are bad, so I don't accumulate files. Sorry, I wish I could have shown them here.
The paint finish was glossy, while my photos looked they were shot on a gray, cloudy day. And the paint looked sandblasted. And formless. It was that bad. And it was certainly more than just underexposure.
The car was facing South. The sun was off the left in the West. I've really only used polarizing filters to darken the sky and reveal clouds, or to remove glare from a plate glass store window. And that was 3 decades ago. I'll certainly be more cautious next time.
I think a 2x ND filter would have given me the effect I wanted.