G'day Vic you'll get heaps of answers dont sand the gelcoat off. I dont know the layout of a Gcat but definately from the inside, I cut the decks of my stingray and put divinacell foam under them then used wessystem to put them back on and bulkheads in the hulls to make a rigid boat. Its a huge job very satisfying but not cheap any photo's of your boat? is it really bad?
regards