I've done some polishing of horn handles after filling cracks with epoxy or superglue. There's always some overlap where the epoxy gets on the horm around the crack and you have to sand it down.
I tape around the crack to minimize the overlap. After the cement dries I use a small, fine file to level off the epoxy or superglue. Then I sand the surface of the area, starting with 400 grit and working up to 2000 grit in steps. By the time you get to 2000 grit the horn is pretty much polished. You can go higher if you want, with polishing powders. After that, some mineral oil or Hooflex cream. (I think some people make the mistake of starting with too rough a file or sandpaper, and that's when you can get tearout.)
It's pretty easy for a small area, but I wouldn't want to do it on an entire handle.