Steps for repairing cracked fenders:

1)Sand down the crack of the fender with 40 or 80 grit sand paper

2) V out the whole crack with a drill or a grinder

3) Cut a piece of plastic off of another fender of the same material and melt it down into the crack(if its not the same material it wont stay together right)

4) Sand down the exsisting melted plastic with 80 or 100 grit sand paper until its flush with the rest of the fender.

5) Put a layer of fiberglass filler(or any other product that will stick to plastic) over the plastic and sand it down until its smooth

6) Spray plastic primer over the fender(so the paint will stick and so when the fender bends the paint wont chip or rinkle)No sanding is required after spraying the primer

7)Then paint the fender