I took it down with 400 and started again. Worked to 2000 grit. Finished with a rubbing compound that's designed for automotive plastic. The stem is nice and shiny black now. If you look closely you can still see pits, or spots. It seems that the vulcanite is very porous. I may just leave it if I can't get it perfect with compound and polish. I don't want to risk sanding off the edges or ruining the profile of the stem. I'll continue to fiddle with it. I'll post pictures when the pipe is finished.