Glass scratches. However, the areas that are supposed to be clear,
cannot get scratched. And working with sand, at high pressure can
scratch glass, that’s really the whole point.
So using a resist, I cover all the glass, (inside and out if it is a 3D object, such as a bottle)
I like to use Buttercut, it may be a bit overkill, but since I tend to
carve through the glass I like it. There is some disagreement about
using buttercut, as it is really for stone carvers, and it is an
expensive material. For some, contact paper is sufficient. The
drawback there is that the stick can be too sticky if left too long.
too that if you blast through a bottle, the sand can go right across
and blast the inside of the opposite side you are working on.
Protecting the inside is tricky. It can at times be best to decide to
blast the inside.
The other thing is that if you are covering a
three dimensional object with a two dimensional material, will require
cutting, and that leaves seams. Those seams are vulnerable, and need to
be protected. Glue… use glue.