Re: Plywood for Floor - Resin Coating
I agree with all the previous posters. Don't thin any sort of epoxy period, and if you're going to thin poly resin use only styrene and not acetone. You should try US Composites 635 thin epoxy resin if you do not intend to use a layer of cloth. It is thin enough and is meant specifically for layup applications where good adhesion and penetration is needed. You can run the numbers on whether it would be cheaper to use cloth plus poly resin or just a thin epoxy resin. Poly resin alone will harden fairly brittle and will not last long on the sole of a boat without cracking so it will not provide the sort of long term barrier coat you're looking for.
As far as the wood goes, pressure treated is to be avoided primarily because of its high moisture content. You want the driest, straightest ply you can find with as few voids as possible. That's what you're getting with marine ply, a dry, straight and clean piece of wood. You're not getting any particular resistance to rot by injected chemicals like in pressure treated wood, just glue that won't delaminate if it gets wet, but you should be getting that same glue in any decent exterior grade plywood. If you let the pressure treated piece sit for a few weeks to a month it will likely dry out some, but will likely warp severely. Also the copper used in pressure treated wood will cause galvanic corrosion with any part of bare aluminum it touches so, at a minimum you will need to barrier coat the top and bottom completely. Many people ignore the underside, which is a mistake. Realistically, with pressure treated wood you will likely be locking in a fair bit of moisture once you barrier coat since a lot of liquid is injected into the wood and doesn't come out which can weaken the resin bond over time. If you can measure the moisture content prior to coating you will know what you're in for but it's easier to just avoid pressure treated all together.
. if the poly had jellyfish in it and was obviously nearing the end of it's useable shelf life, it might require thinning. even in that case i would only add 5% acetone. it will weaken the resin but it will improve its ability to penetrate deeper into the wood. less is more, none is best 
