Polyurethane is a type of varnish that protects wood from water, stains and scratches. Always cover surfaces in your home when you are using polyurethane to prevent it from dripping on glass surfaces. Luckily, glass surfaces are non-porous and you can remove polyurethane because it doesn't stain or damage glass. Use strong solvents that dissolve polyurethane.
Soak a clean cloth in paint thinner.
Rub the cloth over the polyurethane stain on the glass. Work until the stain is completely soft and is gone.
Rinse the area with water.
Soak a clean cloth in acetone.
Rub the acetone-soaked cloth over the stain until it is completely gone.
Rinse the area with clear water and dry it completely with a clean cloth.