I tried to scratch some excess epoxy off my windshield with a pocket knife after a rock ding repair. Take my word for it, never try that. After trying everything from toothpaste to rubbing compound on a buffing wheel, I only succeeded in turning my three one inch long knife scratches into a 4"x6" area of horribly scratched and dull glass
I found this stuff and decided to give it a try before replacing the windshield:
http://www.ebay.com/itm/NEW-Windshie...-/261195721313
This stuff was nothing short of miraculous. For less than $25 and 30 minutes of polishing, the windshield is like new
I highly recommend it!