Observing the glass
Deep scratches in glass can be difficult to fix, and there is a method for testing whether you should even try to fix a scratch that runs too deep. Try sticking your fingernail into the scratch; If it fits, then you have a scratch that is likely too deep to fix.
The reason for this difficulty is that the deeper the scratch, the more probable the glass will break when buffering. Also, the scratch will need to be filled in with Jewelers Rouge (or clear nail polish).
The larger the scratch, the weaker the glass will be.
Tools of the trade
An electric buffer is used to buff the scratched glass surface. Professional glass repair is done with a lamb's wool polishing pad and an application of Jewelers Rouge.
A mask and pair of eye goggles are used to keep dust and glass debris out of the eyes, nose and mouth while buffing the glass. The lamb's wool polishing pad can be used with out the electric buffer, but doing this work by hand will take a bit longer.
A solid, flat surface is used to lay the glass down on while buffering. An ammonia and water mix is used to spray the surface of the glass after buffing.
Some persistent scratches require a layer of nail polish after buffing fails to remove the scratch from the glass.
Applying the electronic buffer to a glass surface is done with extreme care (glass becomes fragile after it has been scratched, especially with deep scratches). Pressing too hard on the glass may cause the glass surface to crack or break completely.
The buffer speed is set to medium to start. Running the buffer speed any faster could break the glass.
After buffing the scratched area of the glass, a solution of ammonia and water (1/2 oz. of ammonia and 2 cups of water) is sprayed over the glass surface.
The ammonia and water mix must not come in contact with the skin or eyes. Plastic gloves are worn to prepare the mix of ammonia and water.
A paper towel is then used to wipe the surface dry. Jewelers Rouge is applied as a finish over the scratched area after buffing (plain white toothpaste will also work).
When the scratch persists even after applying Jewelers Rouge, then the repair can be completed by filling the sheen on the glass with a coat of clear nail polish.