New deck fiberglassing

[giterdone72]

I am getting ready to put down a new deck on my bayliner capri using a good exterior 5/8 sanded ply. I am new to fiberglassing and trying to do this on a budget. I will be using Polyester resin/fiberglass. Is it nesessary or does sanding or 'roughing up' the ply help the glass/resin to bond? is it necessary to glass the underside (bilge side) or the ply or will a good coat of resin work as well for moisture?