Removing devit after slump
This is difficult. Most methods of removing devit involve either a chemical and tack fuse temperatures, or abrasives and tack fuse temperatures. So the heating reauires is above the slumping temperature. Although there are some preparations – bendiong glazes – that claim to give a shiny surface at slumping temperatures, I have never found them successful.
It is possible to sandblast (with fine grit) or abrade with 600 grit wet and dry sandpaper, to get rid of the devit. You then need to use a low slumping temperature (ca. 620C) for a long soak (say 90 mins) to give a satin texture to the surface.
You could coat the surface with a borax solution, flatten it and take it to a tack fuse temp. and re-slump, but this depends on what will happen to the shape when you flatten it again.
Of course, as you now know prevention of devit is better than correction. The glass needs to be thoroughly cleaned before and after each firing. My approach is here: