Even a good automotive paint with a flex agent Will only hold up so good on something that moves as much as a fender. Anything you use as far as a paint is concerned Will only be so good. I'm with Beastly, your best bet is to wet sand the plastic, start with a rougher paper & work finer, once you get the scratches pretty good just buff it out with a good rubbing compound. You can even buff it by hand if ya don't mind using a lil elbow grease. The thing is to make it less noticeable, you can do that with a paint job on a car w/o completely removing the scratch, believe me, I used to detail cars for a living.