For minor scratches and a windscreen not stopping glare but infact expanding the light beam you can cut a potato in half and rub it well on the screen with water, the starch in potato acts as a perfect solvent and will remove almost all oils embedded in screen, use spray bottle and start from one area, do not use lots of water, just wet it and rub it, you will soon see that the potato is almost black from the side being rubbed on screen, use a knife and peel off the blackened potato. One big potato is enough for the whole screen. You will see lots of improvement afterwards, water will almost slip away and wipers will become more efficient and yes glare will be gone too. Remember not to dry rub on screen
