Are Your Glue Joints Repairable?

Discussion in 'Boatbuilding' started by Bob Smalser, Jul 15, 2005.

  1. Bob Smalser
    Joined: Jun 2003
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    Bob Smalser Junior Member

    Have you done a simple builder’s test of your favorite wood glue to see if it can be reglued successfully should either your work be damaged, or a cross-grain glue joint fail with age and seasonal movement?

    In 4 decades as a woodworker, I’ve done a good bit of conservation, repair and restoration work, including pieces in a few federal museums both here and overseas. As I pass what I know down to my boys, included will be what I know about glues. I know that some glue types can’t be glued over, often requiring new wood to be let in during repairs, and the joint recut. I discovered that the hard way some decades ago restoring furniture, and simply switched to other glues for all my work. Since then, those glues I rejected may have been reformulated; plus there are a number of new glues worth checking out, so to make sure I’m not providing bad or outdated advice, it’s time to check out the current crop of wood glues for repairability.

    I make no pretense toward science, here…this is all anecdotal based on experience, not chemistry…all I want to show is whether marine epoxy will adhere to the glue lines or residue of the various wood glues during repairs. You can look up strength and other test data in USDA Forest Product publications; I care about repairability because I’ve never seen any test or even anecdotal data on anything but hide glue in that regard, and it’s important if your work is to survive beyond typical damage and wear and tear over time. I chose epoxy as the regluing agent because it’s the usual choice in professional structural repair work and it adheres to a greater number of diverse substances than any other wood glue I know of. In fact, it usually rebonds a failed but fully cured glue joint much better than the original glue would, and as it also bonds to itself very well, epoxy is a good, repairable choice for many applications.

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    On identical tiles of freshly planed, vertical grain, second-growth Doug Fir, I saturated the faying surfaces with glue and let them cure to full strength by the manufacturer’s instructions for time and temperature….

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    …then I keyed each faying surface with 100-grit abrasive paper, reglued them with marine epoxy, and “clamped” the assemblies to the degree favored by epoxy. For glues that left a rough surface like polyurethane, the epoxy was applied twice…an unthickened coat followed by a second coat thickened with West 404 High-Adhesive Thickener, per the manufacturer’s instructions. I let the epoxy cure for 6 days to reach full strength.

    I purposely chose small blocks of wood with easily broken short grain because strength here isn’t the issue, adherence is, and I can check adherence using a sharp chisel without trying to break long glue joints in a press. Of greater concern was that the glues to be tested were applied without any clamping pressure, but as it turned out, several glues that require high clamping pressure fared very well, so I believe the results are reasonably valid.


    The results offered no surprises.

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    The epoxy thoroughly adhered to the strongest of the off-the-shelf glues, the 2-part resorcinol, breaking completely at the wood rather than the glue line. Attempts to slip the chisel between the glue lines revealed a thorough and unified bond between all three layers of glue.

    Epoxy on epoxy showed similar results…

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    …so did liquid polyurethane (Elmer’s Ultimate)…

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    …and powdered urea formaldehyde plastic resin glue.

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    Titebond, a Poly Vinyl Acetate glue, however, broke some wood but failed the chisel test…. the chisel easily separated the two layers of Titebond, indicating poor adherence of the epoxy in between.

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    Titebond II broke even less wood, with poor adherence…

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    …and Titebond III, while a much stronger glue, still did not adhere to the epoxy.

    The implications of all this can be minor if we are talking about a first-effort coffee table….but they can be serious and even dangerous if we are talking about a strip-planked boat hull made of 1 X 1 strips glued together using an unrepairable glue. Picture the requirement to feather in a large patch to repair hull damage, and you can see that patch will be pinstriped with unsound repair at every glue line, leading to early failure of the repair.

    You can draw your own conclusions. Mine are that the work most easily restored is often the work that survives the longest, that you may not care about longevity, but that may break you granddaughter’s heart some day, and I’d check out my glue choices thoroughly before committing them to any 20-hour high-end project, let alone a 700-hour project.
     
  2. Bob Smalser
    Joined: Jun 2003
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    Bob Smalser Junior Member

    As you might imagine, this draws some concern and even ire among folks wedded to aliphatic glues and other household products for boats, so I’ve done some excursions for them.

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    The only glue I can find that will stick to Titebond residue is cyano acrylate...here, the Hotstuff brand of superglue….problem is, cyano is so durn brittle itself that it is worthless for anything more structural than fixing a sliver. Crossgrain glue joints and anything that gets bumped or stepped on breaks a cyano bond quickly.

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    A number of strip builders these days are using PL Premium poly construction adhesive…I never have, so I tried it…..and epoxy sticks to it….it is repairable.

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    To compare PL to something more traditionally marine, I also checked out 3M 5200….and epoxy sticks to it, too.

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    I also found that PL sticks to PL…

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    …and 5200 sticks to 5200.

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    Call me in a few months to see how these non-marine glues that passed the repairability test fair in the soak test in comparison to resorcinol and epoxy on cedar planking stock…

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    …because I’ve known for years that 5200 loses all adhesion to fully-saturated wood…..here after only a week stuffed under the dock in 60-degree water. Will PL Premium behave like 5200, too?

    Think this doesn’t impact your new boat? Just wander thru your local yards and gander at all the derelicts…coatings long gone…bedding dried out…. sheathings perforated...rainwater saturating the wood unhindered. Your and my boats are all gonna look like that some day. If it has my builder’s plate on it, I want it to be the one chosen for restoration.
     
  3. jehardiman
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    jehardiman Senior Member

    Are you going to publish? I'd like a copy of the raw data. I'll trade it for a yet to be named submersible project.
     
    Last edited: Jul 16, 2005
  4. Bob Smalser
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    Bob Smalser Junior Member

    Yes, JE, I intend to publish this when I'm done. Apparently few builders are aware that some of these popular glues can't be repaired. Of 4 professional strip canoe builders at the Lake Union boat festival, three were unaware of this, and two of those use Titebond. The argument that epoxy and fabric doesn't fail over aliphatic glue lines is valid for a new boat, but not for a feathered-in patch, where the north and south edges at 6:1 or greater will require gluing over a whole lot of glueline.

    I want to hear all the arguments pro and con for possible revisions and excursions before we see it in print. I pulled this off the other forum because a troll got to it and spoiled it.

    Ditto the arguments that I should be doing this as a blind test using a tensiometer in the USDA Peel Test. All that's unnecessary....the tensiomenter would either read 100 or zero, as the epoxy either sticks to the glue residue or it won't....and there's no way for an experienced woodworker either not to bring some bias to the table or not recognize the glues.
     
  5. Bob Smalser
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    Bob Smalser Junior Member

    Here’s an update.

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    In my initial screening, I found cyano to be the only glue to stick to PVA residue – here, Titebond III.

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    I hadn’t factored in clamping pressure. When PL Premium is clamped to PVA residue for 24 hours with high clamping pressure, it adheres well….but the glueline is soft and gummy, somewhat akin to a contact cement glueline….a mechanical instead of a chemical bond that may not stand soaking. It may have potential to repair PVA-contaminated joints, so I’ll do some more tests here for repeatability and with liquid poly and 5200 to perhaps see why this is.

    Will it make a sound repair? I don't know yet...but given the bad alternatives to date, you might try it before recutting the joint.

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    After 3 weeks in 60-degree pond water, the 3M 5200-Western Red cedar bond is beginning to lose adherence even where it received high clamping pressure.

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    The PL Premium also has lost adherence where it wasn’t tightly clamped….but the actual joint remains sound.

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    The epoxy has also lost adherence where it wasn’t clamped…but its joint remains sound, too.

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    Finally, the resorcinol remains rock solid.

    But you can also see from the wood color beneath the lifts, that the wood is far from fully-saturated yet.....it will need a lot more time submerged.
     
  6. Bob Smalser
    Joined: Jun 2003
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    Bob Smalser Junior Member

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    The WRC heartwood test layup that included resorcinol, West System epoxy, liquid polyurethane, 3M 5200, and PL Premium polyurethane construction adhesive was pulled from the pond water after 6 months of submersion, cleaned, and a sample removed from one end and resoaked while the large end was allowed to air dry.

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    There were no differences in adhesion between the sample allowed to dry and the one that remained soaked.

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    Destructive testing showed that all the products tested continued to bond strongly after 6 months of submersion. The 3M 5200 poly sealant and PL Premium poly construction adhesive, when applied without clamping pressure, lost adhesion in saturated cedar…but with a clamping pressure suitable for resorcinol, their bonds remained strong.

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    The “saturated” cedar test layup stabilized at only 24% EMC when corrected for species and temperature. I expected a bit higher reading, but this is consistent with the 30% moisture content of cedar heartwood in the log and the 21% of submerged bilge planking protected by paint. Cedar logs do sink, however, and remain sunk long after their heavier sapwood has rotted away. So this test layup goes back in the water to see if more time will provide a higher reading. I also intend to do another layup of White Oak, which I suspect will rise to over 30% EMC with saturation.

    General Notes on Glues and Goos


    Resorcinol: The marine standard. If you can get 70 degrees F or higher for an overnight cure and consistent and high clamping pressure with no gaps, you won’t go wrong using it. Likes wood at 10-15% EMC, according to Navy tests. Long open time. Repairable with epoxy. Ugly red glue line.

    Marine Epoxy: The repair and restoration standard. Bonds well to a wide variety of materials, and usable in almost all flexibility and temperature conditions. Needs no clamping pressure, only contact…fills gaps well. Likes wood below 12% EMC. Repairable with itself, joints can often be broken apart for repair with using heat. Clear glue line and can be dyed to match the wood. Controllable open time with different hardeners. Slightly permeable to water vapor and there are reports of failures in fully saturated wood and with White Oak. Very sensitive to UV, requiring protection.

    3M 5200: A rubbery, polyurethane sealant in various colors with adhesive properties sometimes used as a glue. Fails as a glue under water saturation without high clamping pressure, and without the proper strength testing I couldn’t do here, it’s not recommended as a stand-alone marine glue. Repairable with epoxy.

    Liquid Polyurethane: Gorilla Glue, Elmer’s Probond, Elmer’s Ultimate, and others. Versatile in temperature and bonding wet wood with moderate open time, these glues aren’t rated for below waterline use but initial use shows potential as a marine glue. Likes high clamping pressure and fits similar to resorcinol…it won’t fill gaps. Will successfully glue green wood at 30% EMC. Repairable with epoxy. Noticeable, yellow-brown glue lines.

    PL Premium Construction Adhesive: This polyurethane goo shows promise as a marine glue with further testing and use. Works like 3M 5200 but cures and behaves like liquid poly. Appears to bond well to everything epoxy does, and more where epoxy and liquid poly won’t, perhaps because of a higher isocyanate content…it bonds to difficult surfaces only cyanoacrylate super glues will bond to. The only general-use glue I’ve found that will bond difficult aliphatic-contaminated surfaces. Appears flexible to temperature and moisture content with gap-filling ability, but as a construction adhesive, its open time is shorter than liquid poly. Appeared to like high clamping pressure, and unlike other glues, wouldn’t bond at all without at least some. Repairable with itself and epoxy. Glue line as in liquid poly.

    Urea Formaldehyde Plastic Resin Glue: Weldwood, DAP and others. The old interior furniture standard, and in older marine applications that required well-blended glue lines. Still preferred by many, as it is a no-creep glue easily repaired using epoxy. Long open time, it needs tight fits and 65 degrees F or higher for an overnight cure…it doesn’t fill gaps. Best glue line among them all and moderate water resistance still make it useful for protected marine brightwork applications. A relatively brittle glue and UV sensitive, it requires protection….but its brittleness is an aid to repairability, as joints can be broken apart for repair. An inexpensive powder with a short, one-year shelf life.

    The Titebond Family of Aliphatics: Convenient. No mixing, just squeeze. Short open times, fast tack, and short clamping times. Fast, and an acceptible long-grain layup glue…in heated, commercial shops, I’ve had rough-cut Titebond panel layups in and out of the clamps and thru the planer inside of an hour. Flexible in temperature and to a lesser extent in moisture content, but the bottled glue can freeze in unheated shops. A flexible glue, it has been reported to creep under load, sometimes several years after the joint was made. The latest “Titebond III” appears to be a stronger glue than its two predecessors. Difficult glues to repair, as they won’t stick to themselves and no other glues will except cyanoacrylates, which are too brittle for general use. Epoxy and fabric aren’t bonding to aliphatic glue lines in marine strip construction, compounding repair difficulties. While not definitive, the new PL Premium appears to bond well to Titebond III residue and is worth pursuing by those repairing old white and yellow aliphatic joints.
     
  7. wdnboatbuilder
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    wdnboatbuilder Senior Member

    very interesting thanks for sharing your work.
     
  8. War Whoop
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    War Whoop Senior Member

    Nice posting and thank you for the information.
     
  9. Thunderhead19
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    Thunderhead19 Senior Member

    Thanks Bob!
     
  10. CapKos
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    CapKos Junior Member

    Excellent work! Many tanks. Did you know more details about gluing white oak with epoxy, especial under water?

    All the best,
    CapKos
     

  11. DanishBagger
    Joined: Feb 2006
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    DanishBagger Never Again

    This is an awesome thread!

    Thank you so much, Bob :)

    Andre
     
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