Protecting Plywood with Polyester Resin

as my great mentor said to me when i mentioned using unsealed marine ply in a floor stringer job, its ****** wood thats sitting in ****** water and its gonna ****** rot. so at our shop we use regular exterior grade 3/4" ply fully encapsulated (top, bottom, and sides) in at least 2 layers of 5oz matt and polyester resin. anywhere there is a screw there should also be silicone. i have also heard about the horrors of trying to bond polyester to treated wood so ive never even messed with it. but i can personally attest to the exceptional strength, durability, and waterproof quality of this method.

 

I can't believe this thread has continued this far. Polyester doesn't seal wood, unless you use so much resin and material you really don't need the wood any more for strength. None of the polyesters even comes close. Vinylester is considerably better, but considering it's cost, not much of a savings compared to epoxy.

Manufactures use poly's for only one reason, their production lines are setup for this resin system. Poly's including vinylester have proven so bad at water proofing, that some production shops have completely switched over to epoxy and all of the high end shops have no choice, but to switch over so they can match the modulus of the resin, to the exotic fabrics they're employing in their laminates.

Most of the hardware store bought stains/sealers and "decking" treatments have a wax base that acts as the moisture repellent. Just try to paint over this and see what happens. They work fine for aunt Millie's back porch or uncle George's fence, but don't employ these in a boat or you'll wish you hadn't. They don't last long on a well used porch, let alone a boat hull or deck. Northern climates are easier on these finishes then southern, but the best of these coatings will not survive long in a marine environment.

All the resin systems we use need UV protection, just like all the plastics we employ in other products. Plastics will de-polymerize from UV exposure. Some are better then others, but all eventually fail. Any one who has removed vinyl siding from a house after it's been there for several years can attest to what happens to these plastics, even those engineered to tolerate UV. The single, cured plastic molecule that comes after the cure, separates into individual components and falls apart into it's elemental segments. When a GRP hull comes out of a mold, it is essentially one giant molecule. With enough UV exposure, it breaks down into the base molecular pieces, just like all plastics.