New blade = no scratches
A razor blade removes paint overspray and gunk. Keep the glass wet and use a new blade each time. Microscopic rust particles on the blade can scratch the glass.
Handy Razor Blade Storage
It's convenient to keep extra utility and straight box cutter blades near your workbench, in the kitchen and in the garage. But the question is how to store them when they're not in use. A good solution is to glue a magnetic business card (a refrigerator magnet) to the inside of a cupboard door and to your workbench with the magnetic side out. The magnet is strong enough to hold the blades in place even if you slam the cupboard door.