Over the years I have tried Sand blasting (far too coarse!!) bead blasting (better but still too coarse) valve grinding paste (better again and now getting into the 'usable' range) and orbital and vibration sanders using aluminium oxide (better again if you can avoid hot spots and unevenness).

None of them give nearly as good results as using some fine powdered grit and using a glass block as a grinding blank. Find the instructions that Ian Grant posted here on APUG.