Yes sandpaper needs to be secured to glass, spray adhesive handy for this. As I do this I make sure to rotate the casting so the sanding goes as evenly as possible. Go SLOWLY and check often using firm finger pressure. Did I already say check often. I would not use anything courser than 340 Grit, 400 is a good choice I think.
I accidentally broke my glass recently, a scrap .5 inches thick. I had had 20 years.
