Hi Susan, Paint thinner will have no effect on dried paint, as you have already experienced. Get a can of paint stripper, preferrably a gel, to keep it from running down the window. Follow the directions on the can, and you will dissolve the paint so it may be wiped off. You may have to follow by wiping down the window with paint thinner to remove any residue of the stripper, or just hot sudsy water. Best of Luck.