He has hard chrome and a black polymer finish. I always get the extractor done in the polymer, and also did the decocker buttons in that on my P99s.
But, I wouldn't do an entire slide in it - he couldn't get it right on a 1911 slide I sent him 1.5 years ago.
For black refinishing - I'd go wither with Birdsong's Black T or CCR's cerakoat.