Cored decks, like cored doors get their stiffness from the outside laminate layers, not layer.
You'll want to put at least one layer of FG over the core.
Imagine a door to your home with just the outer face and some cardboard waffles on the back. Imagine how fast it would warp out of true.
Another point, check your epoxy's curing temp's for min/max. Gooey don't work.