Repairs to plastic roto-molded boats are done with a thermo plastic welding rig. This is a device that uses a special hot air gun and filler rods of compatible plastic material, very similar to a wire feed arc welder.

http://www1.mscdirect.com/CGI/NNSRIT...MT4NO=57654184

Fiberglass will NOT stick to the plastic, so a gel coat type repair is not possible.

A local rafting/kayak shop would have the right equiptment. If the scratches are only cosmetic (ie. they don't leak!) I wouldn't be too concerned. The advantage of the plastic boats is they can take a lot of abuse w/o leaks, the down side is that nothing sticks to them.

I have heard of some folks using rough sand paper to smooth out gouges and then using a propane torch or high temp heat gun to singe off the resulting hairs.