Run a fingernail across the damaged area. If the nail drops into a groove, the scratch is considered deep and may require professional repair; however, if the nail skips over the surface, the scratch is considered light and can easily be repaired at home.
Wash the affected portion of the window in a solution made from 1/4 cup isopropyl alcohol and 1 cup water. Dip a soft cloth in the mixture, then rub the surface of the glass to remove any dust or dirt.
Measure 1 tbsp. ground mustard seed into a small bowl. Add 1 tbsp. distilled white vinegar and stir well. Spread the vinegar treatment evenly over the scratch, then allow the paste to dry.
Buff the vinegar mixture lightly with a dry, lint-free cloth to remove the paste. Work carefully using small, circular motions, applying pressure only where needed. Mustard seed is mildly abrasive and grinding it into the glass can lead to more scratches.
Re-wash the window with the alcohol solution, then fill in any remaining nicks or cracks with clear nail polish. Brush the polish over both sides of the glass, sealing the crack in a thin layer of varnish. Allow the polish to dry, and then repeat, if necessary.