Scratch repairs on plastic rim?

Today I had the misfortune of reversing into a metal fence and the rear end of the car is badly scratched. Some of the (black) paint has been badly removed and there are some deep scratches. I tried to repair the paintwork with a touch up kit which improved it slightly but doesn't look great. I want to know how to get rid of the scratches on the plastic rim of the rear bumper? Thanks.