Guys, (and gals) I found this website today, maybe it has already been posted before(but did not find it on the forum), but it is a wealth of knowledge if you plan on modifying/decking your jon boat.  It also has links to other sites that have even more information.  

It is gonna help me a lot this winter as I deck and modify my boat.

http://secretweaponlures.com/odf/odfsite.html

Just a quick point.  You're much better off using epoxy resin to seal plywood than you are fiberglass resin.  Fiberglass resin is kinda brittle stuff if it's used in a stand alone situation like layered on a sheet of plywood as waterproofing.