I agree with one of the other posters. I have been in collision repair for years and have never seen anyone lay primer down first and follow it up with filler. If you have a good scratch on the bare metal that will be the best base for the filler to grip and stick to. Otherwise you risk the plastic shrinking and leaving funny looking alien paint circles under your new paint, or worse, popping loose completely. If you're doing a complete paint it would be a good idea to do a coat of sealer on the whole truck, especially if you're spraying a more transparent color, a little more uniform finish when you're done. Good luck, take you're time, and have fun.