If your using a Glass call and sanding them, thats where the problem is starting that your having such a hard time.

Do Not sand Glass, go get yourself a conditioning stone and you'll need it for a good quality glass anyway. When you work the area with it Only do it in one direction and only 3/4's of the glass surface. This will help you in very low light see what direction your working the glass if you need to give it a tune up.

Sand paper does not get down in the surface of glass, as stone does...