To remove a scratch from a car's windscreen, liquid acrylic scratch remover or a paste made of cerium oxide powder can be used. Deep scratches, however, cannot be removed completely.
To use liquid acrylic scratch remover, apply the liquid directly onto the scratch. The liquid spreads into the deepest parts of the crack in the glass, and then becomes nearly invisible upon hardening.
To use the cerium oxide powder, mix it with water until the substance gets about as thick as craft glue. Then, buff it into the scratch and the immediately surrounding area of the car's window glass.