The recommended material was Cerium Oxide, which I got from a local lapidary shop. As used in lapidary it would probably most often be mixed with a bit of water to make a slurry and stones would be tumbled (for hours or days or weeks).

I spent quite a few hours with it, but with a large flat surface like I had it was difficult to get very aggressive without flinging the slurry around the living room (i.e with big power tools). Concentrating on small specific areas, I wasn't able to detect any effect on the glass from about 20 hours of aggressive working on one 3 or 4 inch square area.

It likely was not a bad choice for a polishing agent; but the "project" was too large for my tools, available workspace, and energy. It might have worked better placed between two pieces of glass under moderate load (to get higher, even, contact pressure) as used in telescope mirror finishing; but another piece of "similar glass" would have been another $125 and wouldn't have needed "finishing" to be used as a replacement.

It was an experiment to satisfy my curiosity.

Anyway, I've got a sufficient quantity of the stuff if I ever do decide to grind my own telescope. I'd likely set up machinery for that.

John