i guess it depends how bad it is, mine had a 6" x1"patch that had flaked and crumbled, and it had also gone badly at the edge where it goes over the door opening. I used a combination of milliput and ordinary plastic modelling filler after 1st thoroughly cleaning/degreasing the piece. Once everything was sanded and blended I used SEM texture and SEM colour. The end result was ok for a driver if you didn't look too carefully, but as I just wanted to pretty mine up until i got a replacement that was ok. The biggest problem was getting filler to adhere to the edges where it was really thin, perhaps using superglue to seal it prior to filler may help.
To be honest its a lot of work and you would prob be better off looking for some better ones in a scrap yard even if you have to paint to match, or get some replacement repros,
good luck anyway
Steve
