Sealing floor? Should I CPES inside and will it affect .....

Building my benroy on 4x8 HF frame, built my floor a month ago and it was way overkill! 3/4 ply wood x2 and not great wood, no worries I made a new work table out of the heavy bastard!!! Now I'm making a 3/8 sandwich fool with foam in the middle now. I will cover the bottom with Henry's roofing tar, my question is do you guys seal the wood first and what about the inside where the foam will be? Any help would be great! Thanks Adam in KC