Just remember, when you grind off the gel coat of the hull you will have to replace it. The gel coat is what keeps the hull water proof. when the hull is made it starts with the gell coat painted on to a mold then the GRP is layed on topof the gel coat. You can get Flow coat which can be painted on and has the same properties as gel coat (a waterproof wax and other chemicals added to the resin) from most roofing supplies, you may have to add pigment to it to match the hull, but that is not a big problem.
Good luck.
TC
Oh right didn't know that. I was roofing up till October last year