The Tuff Coat bonded very well to the FRP - when I smashed through the deck dropping the trolling motor much of the busted fiberglass was held together by a Tuff Coat skin. I rolled it on, only because I would have had to buy a hopper sprayer to install it that way, but I would have preferred to have sprayed it. I only applied it up in the front, so I haven't seen it exposed to gasoline yet. Guys on my CT Fishermen forum used it on the entire inside of their Boston Whalers, and it's held up there for them, so I'm sure it's been exposed to gasoline in their applications - not that anecdotes are good for much. If you should call the manufacturer and ask them before I do please let me know - I'll do the same.
I haven't finished in the back of my boat, but I will have some rigid foam atop the rear bench when all is said and done, but not over the space between the transom and the bench (that's getting an aluminum plate as I can't back it with foam.) Most of the foam I'm using is the 2 part urethane, but I will need some rigid to support for the deck above the livewell that's going in the rear bench, and pour foam won't give me any future access to the livewell without making a mess...I'm planning on a layer of the thinnest extruded polystyrene I can find, a layer of FRP, another layer of poly and my deck layer of FRP. I've got 6 pieces of aluminum square tubing running parallel to the keel atop the bench for my seat mounts to tie into, so the foam/FRP layering will be in narrow slots between these ribs.
I've heard of people using latex paint to seal the rigid foam and protect it from gasoline, though one scratch and that is for naught - though I can't come up with anything better so I'm probably going to do the same and hope for the best.