The trick with a belt sander is to use black silicon carbide belts, the kind actually distributed thru glass outlets. 3M makes them. Otherwise use a sanding block with fine
black emery cloth tightly attached to it. Sorry Steve - I thought it was you Brits who didn't know what power tools are. So I won't say exactly how I trim down a groundglass,
because I have certain, uh, logistical advantages, like a forty-page list of German power
tools warehoused fifteen feet behind my desk.