Use bolts with beveled (rounded heads) to secure the board to the frame with a nut underneath the frame. Around here lumber yards sell treated wood that is outside so its heavy and wet you try use it you will have shrinkage between the boards.For the application of the resin to the underside of the floor get the trailer up as high as you can and safely support it. I am getting close to putting my floor on my trailer and my side walls up. Find the perfect utility trailer tarrant al product at VEVOR. ⚠️ Always ensure you're fitting Genuine Parts—quality you can trust. Available exclusively through us and our Approved Distributors. I've used polyester resin before, which will seal it water tight. It might be a bit expensive depending on how much you are doing, but it is water proof. I would assume that solid wood PT boards would be stronger and last longer but I'm getting conflicting evidence online as a lot of people recommend plywood.