What to waterproof plywood with?

Discussion in 'Boatbuilding' started by Ward, May 15, 2003.

  1. Ward
    Joined: May 2003
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    Ward Junior Member

    Im currently fairing my pirogue, but its getting pretty close to done. I'm just curious what I could use to waterproof the plywood with, both above and below the waterline. I was going to use polyester fiberglass resin (bondo brand) and just brush it on like paint, then sand it smooth and paint it. I was just wondering if there is an easier way to do this. Home Depot sells an epoxy paint, both spray on and brush on. Would this work? I have painted engine blocks with this stuff (the spray on), and I do know that it is tough stuff. I am not sure however if it would adequately seal the plywood, especially below the waterline.
  2. gonzo
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    gonzo Senior Member

    That epoxy paint works just fine. However, remember it is not UV resistant, so if it is going to be outside all the time it is better to coat the epoxy with an enamel.
  3. Guest

    Guest Guest

    Waterproof plywood should be used to start with. Then, overcoat with epoxy paint coat to seal the surface. There is a lot of cost difference in the base material. Besides, the plywood should be treated to seal off the core material.

  4. Ward
    Joined: May 2003
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    Ward Junior Member

    Well, my goal was to do this thing cheap. I used stadard (non-marine) 1/4" plywood. I however did paint the entire hull with fiberglass resin, which seems like it will seal very good. For further protection, I'm going to sand the resin smooth, and paint with either epoxy paint, or a tough enamel. I believe that will seal the wood adequately, but I could be wrong. Has anyone else done something like this before?
  5. flame
    Joined: Mar 2003
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    flame Junior Member


    I chose epoxide to protect the plywood (applied using a flow-coat technique, sanded with 80 + 120 + 240 grain size), plus a finish of 2-component polyurethane paint, partly pigmented (3 layers), partly transparent (2 layers inside, 6 layers outside)


  6. stiletto
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    stiletto New Member

    I have a Glass over plywood trimaran and have replaced the decks with construction ply which I have put epoxy resin on both sides in a similar fashion to what you have done. When they are finally fitted to the boat I will cover with glass cloth and more resin, finishing with a deck paint.
    Some earlier temporary repairs I had made with almost exactly the same treatment you have given your plywood proved very hard to remove to make way for the new job and were very durable. The epoxy paint will provide good sealing to your hulls both above and below the waterline. I would stick with epoxies as that is what you have used so far.
    Remember that painting is a system and all elements of that system should be compatible. The paint manufacturers offer good free advice in their technical literature and will be able to specify the most appropriate product. Whether the boat will be kept in the water or not could have some bearing on it.

    Good luck
  7. Guest

    Guest Guest

    Ward...You're gonna have....

    a mess on your hands if you try and do w
  8. Guest

    Guest Guest


    Ward...if you've already used polyester FINISHING resin to seal the plywood it's already too late to change.

    You should have used polyester LAMINATING resin thinned with either M.E.K. acetone or styrene monomer for the first 2-3 coats. This thinned resin really soaks into the ply.

    Once you're sure you have 100% coverage you should apply one layer of fiberglass cloth with unthinned laminating resin, final coat is with the finishing resin and then your paint of choice.

    Which ever paint you use make sure it's catalyized or you'll be sorry...good luck.
  9. Guest

    Guest Guest

    Polyester resin on plywood is a bad idea. It doesn't bond well with anything other than itself really(and even then only after a good sanding). This is why its used for building boats with no other materials other than cloth.
    Epoxy however is a far better "bonder" and should always be used when you want a resin that will stick to something. It can be more brittle than polyester and that is the reason it is rarely used to make a "glass" boat with.
    I had a Glen-L Rampage jetboat with polyester resin/cloth over ply that I bought to strip for the hardware. In the few times I drove it before stripping it the glass was coming off the bottom in sheets. It was delaminating all over the place. Polyester resin BAD!
    My Rampage has west/east system in and out and is holding up to 75 mph blasts just fine(for the most part).
    Epoxy is THE only choice here IMO
    Cheers and good luck.
  10. Guest

    Guest Guest

    Please don't take offense...

    but if you boat was litterally delaminating it's because you either had a substrate problem or I'm sorry to say you don't know how to glass.

    I appologize for questioning you ablilities but after buildling a number of small wooden boats, kayaks and speed boats (all out of wood) I have never once had a delamination issue using polyester resin.

    Properly applied Polyester is a perfectly good material to encapsulate wood...any wood.
  11. Tim B
    Joined: Jan 2003
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    Tim B Senior Member

    I'll second that, I'm sailing a LARK, fully GRP with polyester it's about 20 years old, and other than correcting a slight break inside and a trailer-made gash (which had been left for years) on the outside, I've done very little to the hull.

    Polyester resin is fine, Epoxy tends to flow more smoothly and is good for finishing. Don't whatever you do use polyester and epoxy one over the other. Even if you do get a smooth finish, it won't last.

    An old trick from the Aeromodelling history is to use thinned non-shrinking dope over the bare ply, this sinks in and gives a good hard waterproof finish. It's only a few dollars a tin anyway. Then just finish with two or three coats of epoxy. More coats - Harder finish ; Fewer Coats - Less Weight.

    Best Of Luck,

    Tim B
  12. Guest

    Guest Guest

    Polyester resin makes a very poor waterproofing finish if the boat is going to be left in the water for extended periods of time. The resin will allow water to penetrate the wood and cause the wood to rot. Have you ever heard of hull blisters on gell coated polyester resin fiberglass boats? The reason most of the boat manufacturers are going to "wood free" manufacturing techniques is for that exact reason. Epoxy resin is the way to go.It is completely waterproof, tougher, and there are a lot of epoxy/plywood boats out there that have seen extended service for years and the epoxy/glass applied over the plywood is still in excellent condition. The only drawback is that epoxy is more expensive but its worth the money and you can apply epoxy over poly but not poly over epoxy. Hope that helps,

    Tony Hughes
  13. gonzo
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    gonzo Senior Member

    Polyester resin absorbs about 15-18% of water. That is bellow the percentage necessary for wood to rot. The proble is with water intrusion through delamination and unsealed holes. The blistering is an completely different problem from waterproofing. There are many causes for it, but it is mainly a chemical or hydrolisis problem. Epoxy is a better adhesive, hence less delamination. It is also a better sealer because it absorbs about 3% of water. The downside is the difficulty of working. Another solution is vinylester resin. It works like polyester but has better adhesion and water resistance.
  14. Reese
    Joined: Jun 2003
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    Reese Junior Member


    Are you sure of the 18% figure...never heard of or seen any figures that suggest fiberglass with polyester absorbs anywhere near 18% water. If that were even close to being true it would seem improbable that we would have so many waterproof chemical containers made out of polyester.

    Most of the folks pushing epoxy (don't mean gonzo) are generally also selling it as well...not much profit in polyester.

  15. gonzo
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    gonzo Senior Member

    Yes, I'm sure of the percentage. Try a moisture meter above and below the waterline. It doesn't affect the laminate, it is normal. There is a really good article about it in the last "Professional Boatbuilder". They also explain Hydrolisis in detail. Another way of determining moisture content is weighing a sample before and after oven drying it.
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