How about pulling out the part, get some 600grit wet dry sandpaper, get rid of the seam, and start finishing it with higher grit polishing cloths, 1200,2000,4000, and higher to get a mirror finish.

Stewart Mac Donald website for instruments has the polishing cloths that i speak of. I use 4000 for polishing the feed ramps on my 1911's.

MAKE IT YOURS.
Chuck