Polyester laminating resin is always sticky after curing. It is designed that way so the hotcoat will bond better. All you should be sanding is the lap line to get it close to flush with the foam on the deck and the fiberglass on the bottom. Your sandpaper will always gum up when sanding laminating resin, it's just part of the process.
Adding surfacing wax to the laminating resin is for hotcoating only.