If you have a scratch on an acrylic crystal, don't be tempted to polish just the scratch as the depression that it will leave will catch the light and look awful, do the whole of the crystal evenly with the afore mentioned polywatch/brasso/ toothpaste/solvol autosol then it will look fine..
if you have a scratch in mineral glass, the job becomes a whole lot harder
, it will take around thirty minutes and your fingertips will feel like they're ready to drop off!!!
the 'proper' way to do it would be to use a nasty substance called Cerium oxide (really bad for any life form if inhaled!), the way i did mine was to place wet and dry paper on a sheet of glass (for the 'flatness' or level surface ), i then wet it with water/fairy liquid, held the watch crystal down and went back and forth.
I work as a burner in a heavy foundry and it's hard on watches, here's my old work watch (it was getting hard to tell the time with it
the burn @ 9'clock was very deep and wouldn't come out unless i had a lot longer at removing it..
once i had the crystal more or less evenly opaque, i used newspaper with brasso poured on and again used the back and forth rubbing method, i kept buffing up the crystal with a clean cloth until i was satisfied with the result (actually, my fingers were too sore to continue
i forgot to say that i laid the newspaper on the sheet of glass as i did with the abrasive paper;)
I also had a polish at the case with the brasso....
Good luck with your watch mate;)