I was replacing some phillips screws with slotted on my L-4 and as I was tightening down one of the last ones, the screwdriver slipped and put a nice 6" gash right down the middle of the center window of my upper door. I didn't want to spend the money on an aviation repair kit, so I tried an auto parts store. There were several to choose from, and I went for the cheapest. Turtle Wax has one for about $8. They're meant to repair yellowed or scratched headlight lenses.
It worked beautifully. Took me about 20 minutes and a sore arm, but the scratch was a bad one, and you can't even tell where it was now. Just follow the directions and you can't go wrong.