If you have and scratches or
If you have and scratches or surface irregularities (and you probably do), starting with 400 grit is going lead to a lot of unnecessary work (i.e. an exceptionally long time with that grit). Starting with something coarser — like a 120 grit — will require extra steps (at least a 220 grit before the 400) but the overall time will be significantly shortened.
Helios Kiln Glass Studio