In article , Matt wrote:
I'd probably try a selection of coarse abrasives/carbide sanding
plates in a 1/3 sheet orbital sander or maybe a tungsten carbide
sanding plate in a Fein Multimaster.

As Andy Burns points out, ATMs (Amateur Telescope Makers) have
been doing this for years. They specifically do it by hand because
there's a significant chance of causing a hotspot in the glass if you
use a power tool, which would distort the glass (they're working to
sub-micrometre tolerances), and significantly increases the chance of
spalling off a chunk.
Just use the carborundum, by hand. It's no more time-consuming
than honing the edge of a scythe.

--
Aidan
Aberdeen, Scotland
Written at Mon, 20 Nov 2006 07:20 GMT, but posted later.