sealing ply

Discussion in 'Fiberglass and Composite Boat Building' started by Anatol, Dec 2, 2015.

  1. Anatol
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    Anatol Senior Member

    Hi all,
    I know the Gougeons, etc recommend sealing ply with epoxy in stitch n glue builds. I see the sealing, but wonder if, on surfaces which will not be skinned with glass/epoxy, and will not be normally in contact with water (ie interior surfaces) whether polyurethane or some other newer, cheaper or less noxious/morte convenient sealant/paint is advised. The goal, I understand, is to have it soak in and permeate the wood fibres. Do spirit based products do a better job of that than water-based (swelling issues aside). I'm thinking perhaps acrylic emulsion? Bonding on both sides is a big question. I know nothing will stick to dry PVA.
    thanks!
     
  2. rwatson
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    rwatson Senior Member

    First, not covering plywood with epoxy that is above water will save weight, but may introduce more problems.

    The biggest advantage of epoxy over ALL surfaces is that it prevents moisture entering the plywood at all, whereas having just varnish or similar doesnt give the same quality of protection for a plywood boat hull.

    Epoxy is superior to any other kind of surface preparation, "spirit" based or not.

    Given that a big percentage of plywood boats rot out due to water inside the boat, especially rain water that comes from above, having only exterior protection may be counterproductive. There is a good case of just doing the inside in epoxy if you HAVE to skimp, as the outside of the hull will dry much better in the fresh air.

    Whether it is less noxious may be a question of whether you are sensitive to Epoxy formulations.

    Finally, there are lots of comments about the fairy tale of Epoxy "soaking in" to marine ply. If you look at tests with different types of epoxy, the depth of penetration and edhesion is often done on cardboard, solid timber and even newspaper.
    Testing on plywood is pretty pointless due to multiple laminations separated by the glue lines. Even end grain on marine plywood doesnt absorb much epoxy.

    Also, the more viscous epoxies offer less water protection, as studies and tests show.

    eg.
    http://www.epoxyproducts.com/penetrating4u.html
    and

    www.ewoodcare.com/Epoxy Penetration Test.pdf
     
    Last edited: Dec 2, 2015
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Don't spoil the ship for a ha'penny worth of tar !
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Comparing conventional paints on wooden substrates, solvent-thinned Alkyds fall away from rotting timber very easily, and have the virtue (?) of at least showing you where the rot is, whilst acrylics can create the impression of soundness by maintaining film integrity, while a multitude of sins is hidden underneath it. Nothing is waterproof in the world of organic coatings like epoxy, that I know of.
     
  5. Poida
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    Poida Senior Member

    Very true Mr Efficiency.

    The hull on my boat (wooden) seemed sound to me, I nudged the jetty as I was coming up to the boat ramp. Water sprays started coming up.

    By the time I got my car and trailer to the ramp the boat was about 1/3 filled with water.

    I only just retrieved it damaging my clutch in the process.

    When I ripped away the fibreglass and coatings the hull was only 3mm 1/8" thick which of course had cracked.

    I'm still confused as to what is the best way to treat ply.

    Poida
     
  6. Anatol
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    Anatol Senior Member

    thanks!
    Epoxies sometimes have fillers, and sometimes solvent/thinners. Are some epoxies - by formulation - thinner than others? - as opposed to addding thinners? And if thinning is acceptable - what thinner, in what amount, is recommended for sealing ply?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    You can always put a less viscous, and easier to brush in, coat first, followed by another coat with more body to it. That's the way I'd go.
     
  8. PAR
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    PAR Yacht Designer/Builder

    Anatol, I'm not sure where you're getting your information, but you seem a bit misinformed about epoxies. Marine epoxies (the two usual formulations) are 100% solids, no solvents, no fillers, just a resin and an activator (hardener). You also seem to think improving penetration will be helpful. Yes, some resin brand are less viscous than other. RAKA is known for being watery so, but additional penetration isn't as helpful as you might think.

    This said, you can get a "pre-mixed" epoxy, that does have other stuff in it, such as UV inhibitors, fillers of various types and even solvents, but all of these are special use formulations and not the typical marine grade stuff, used to wet out fabrics or bond hunks of wood together. Some of these come with their own dispensing devices, like double tube caulking gun cartgages. This can be convenient for some, but it's a lot more costly, compared to mixing up your own.

    Do some research and avoid the "hype" of the "penetrating epoxy formulators", as for the most part, it's just snake oil they're selling at a very high markup BTW. The are many previous threads on this site about epoxy and epoxy use. Of course, the major brands also have use guides and even free download books on the subject.

    In a nutshell and to save the bother of a night of reading, you don't need to encapsulate plywood, but it's always recommended. You could just tape the seams (requires epoxy) and paint the surfaces. Many work boats are done this way, but you're losing the advantages of encapsulation and sheathing. Simply put, encapsulation seals the plywood, which prevents it from absorbing moisture, which lets it live longer. Additionally, a 'glass sheathing will protect the surface from abrasion and offers additional waterproofing. Polyurethane, alkyd and acrylic paints will not seal the plywood (far from it), so epoxy is still the choice for best protection.

    Maybe it would be helpful to tell us what you're looking to build. Cutting costs is always a good thing, but certain material combinations need each other to work well.
     
  9. Anatol
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    Anatol Senior Member

    Hello Par,
    Thanks for that. I've done a fair bit of research over the years, including report on testing Smiths penetrating epoxy. Which, as I recall, the tester concluded, was resin with solvent.

    "Marine epoxies (the two usual formulations) are 100% solids," Now I'm confused - if they were 100% solid, they'd be...solid. (not liquid).

    "You also seem to think improving penetration will be helpful." Intuitively, having it soak in seems better than adhering to the surface. But, I'm here to learn. Your use of 'encapsulation' suggests penetration is a red herring...?

    So you're saying, no additional penetration is necessary, so no solvent is advised, in this application?

    Just out of interest, what is an advisable not-so-toxic solvent/extender for epoxy?
     
  10. PAR
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    PAR Yacht Designer/Builder

    Being "100%" solids, simply means nothing leaves the goo during the cure. Paints are much less, some as little as 20% solids, because the "vehicle" (solvents) evaporate, leaving behind the cured resin. A really good paint might be 50% solids, but there's no such thing as a 100% solids paint.

    As with 99% of the "penetrating" epoxies, solvents are used. Smiths is about 67% solvent, 33% actual epoxy, so you can just imagine how waterproof this stuff actually is (it's not). The testing of these types of goo's has discovered, that penetration isn't what offers the waterproofing, but it's the quality of the coating (regardless of penetration depth) that determines how well it works. So, a surface coated with a product that leaves 100% of material in place when cured, is a lot better than one that leaves 33%, even if the 33% stuff has deeper penetration. Yep, "penetration" is a marketing tool and little else. On a technical level, you can improve peel strength and the modulus of coating with a penetrating epoxy, but testing (again) has shown, you only get a few percent improvement, which to me isn't enough to warrant a different pre-coat product under the usual suspects.

    I don't advise you fool around with epoxy formulations (solvents) without a really solid grasp of the chemistry involved. For example, if you dilute epoxy with say denatured alcohol by only 15%, you'll get a water thin, runny goo, but it will also have only 50% of it's tensile and compressive strength. This is why you shouldn't screw around with goo formulations, without knowing what's going to happen the physical properties of the cured goo.
     
  11. rwatson
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    rwatson Senior Member

    If you consider what it is that the epoxy is adhering to, it might re-assure you a bit more. Consider the top veneer of plywood. This is the only thing that can be "penetrated", as the layer of glue underneath is as far as epoxy of any type can penetrate. So, for a start, ANY coating, is only as good as the first layer of plywood glue.

    This veneer, made up of multiple closed and cut cells and fibre, supported by long strands of lignin, (which provides the "strength" of timber) is your major mechanical foundation, and there is no point exceeding that.
    http://www.ili-lignin.com/aboutlignin.php

    Even if you could pour liquid steel over the wood with small spikes extending into the timber it self, as a silly example, sufficiently heavy impacts could STILL fracture the wood/surfacing bond as the wood layer below the mechanical bonds , OR the first plywood glue line fractured. So, there is no point trying to overengineer the join line.

    Back to reality.

    Consider that the cell walls in softer woods are from about .7 to 3mm long, so when they are cut, they provide a terrific mechanical bond for epoxies.
    www.ce.berkeley.edu/~paulmont/CE60New/wood.pdf

    If you measure the bond strength of the epoxy to cell wall to lignin, its easy to determine what a great bond it makes, and the cells are big enough to provide a great "tooth" for epoxy. If you wrench a layer of fibreglass off marine ply, the break is largely where the wood cells fracture. You will see huge patches of wood adhering to the underside of the glass sheet.

    Even IF you first put a diluted epoxy over these open cells , you are introducing a much weaker epoxy layer between the wood and the "full strength" epoxy. Depending on the formulae of this "diluted" layer, it can easily break before the tough wood structure. Micro fractures over time within this weaker layer, provide a potential moisture trap.

    A trick I learned from these forums makes the proper bonding to timber sooo much better, and improves ease of application dramatically.

    As you apply the full strength epoxy to timber or just wetting out the glass, use a heat gun set to low, to gently warm the epoxy. You can see the timber is much clearer to view, indicating a closer bond with fewer air voids at cellular level.

    This will provide ALL the strength you need - equal to the strength of the veneer - which is all you need. AND it will be properly waterproof when enough epoxy is applied.


    Solvents may be less toxic - but they make the ingress of epoxy into your skin much easier. Once cured, and coated, epoxy is not toxic for normal use. If you take the proper precautions while applying, the chances of getting poisoned are very small. If you use a "thinned" epoxy, not only are you reducing the physical characteristics, but you are using a product more easily absorbed by your skin.
     
  12. rwatson
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    rwatson Senior Member

    Assuming you were using epoxy, then if the glass "cracked", you had an insufficient strength of glass, or plywood, for the impact(s).

    Even a small crack will allow water, therefore rot, to progress to complete wood failure.

    A good example of how important complete waterproofing wood is.
     
  13. Anatol
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    Anatol Senior Member

    PAR
    got it! thanks. Great point about solvents and toxicity.
     
  14. Anatol
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    Anatol Senior Member

    rwatson
    great explanation, thankyou, and for the heat gun tip!.
    The cell/fiber tooth image is especially useful - if obvious , if one bothered to think about it :)
    And the veneer thikness issue. Of course! if its marine ply of a good grade, the adhesive will not be porous -to anything!
     

  15. Anatol
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    Anatol Senior Member

    viscostity and solids

    so one wants 100% solids - in the sense that all the liquid becomes solid, and doesn't evaporate off - and one wants least viscosity. What brand in US does this at the best price point? I've heard talk of Raka but have not used it.
    thx
     
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