Wednesday, February 17, 2016

Time To Laminate Some Plywood

Here's where we are now...first layer of 1/2 inch ply glued and screwed to the bulkheads and stringers.  Just some cleanup to do with the belt sander left, and I still haven't put on the aft transom yet.  That's a quick job though.


So now I have to make a decision as to how I will laminate the subsequent two layers of 1/2 inch to the bottom, and the one layer to the sides.

The safe and sure way to go of course is to use epoxy.  But I will be using epoxy for sure as a final layer with either Dynel or Xynole cloth, and that will be expensive enough.  Do I really, really need to use epoxy to laminate the ply together - especially since it will all be well protected from moisture (barring collision damage) by the epoxy/cloth barrier?

In a previous post I documented a test I did with Titebond 3.  It is strong enough without a lot of clamping pressure (in the test none at all) to exceed the strength of Douglas Fir plywood across the grain.  In other words, when I broke the sample apart it was the plywood that failed and not the glue joint.

However, it was generously pointed out to me by Dave Z that voids can be a problem.  So I think I may have come up with a way to deal with it.

If you have been following the blog I indicated that I will nail the laminations with Raptor polymer nails.  They are quite strong in tension, don't rust since they are plastic, and can be sanded and cut with power tools.  So they have very distinct advantages when used in a marine situation.

What they won't do however is act like a screw, and pull the ply together.  So I need to apply pressure somehow at the location I wish to fire the nail, fire it, and move on.  I thought of driving a screw and extracting at each nail location but that would take a ridiculous amount of time, and also would perforate the plywood badly.

The plywood probably should be nailed every six inches.  If I include nails at the borders of a 4 by 8 sheet, then I need 153 nails per sheet, each nail being fired in with that location clamped somehow.


It would make sense to draw the grid on every sheet.

Most of the ply is being nailed down on a horizontal surface so applying body weight makes sense at each location on the grid.  A knee won't do though.

Here is what I'm thinking:


This would be a piece of plywood with a one inch dowel glued into it, which I could strap to my chest.  If you can now imagine me crawling around the bottom of Autarkia, placing the end of the dowel as close as possible to my nail target and leaning into it hard, I figure I can get almost half my weight on to that one localized spot before driving the nail.

I could likely get a nail in every five seconds so my sheet would be fastened down within 12 minutes.  The question is do I have enough working time with the Titebond to do that.

Another test is in order.  I think I'll make up a 3' by 3' sample, apply the glue and wait ten minutes before leaning/clamping/nailing on the six inch grid.  I'll let it dry for a day and then saw it up into 1.5 inch strips and try to get them apart.  If I'm happy with that test it will be a go I think.  Stay tuned.

10 comments:

  1. This comment has been removed by the author.

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  2. sounds like a job for gorilla glue.(it fills voids) and perhaps aluminium screws? or brass?

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  3. Clever idea, but it makes my chest hurt just to read about it! 8)

    I'm with Everitt... Gorilla Glue works great (we used about 1ft pattern with it). Another friend preferred subfloor PU glue by the tub, spread with notched spreaders.

    One possibility might be to thicken the TBIII with something fine (woodflour, talc, bleached white wheat flour, etc). You'd likely want it thinner than putty? Shorty Routh uses thickened TBIII on PDRacers as the primary fillet/weld (not even tape). Our experiments have gone pretty well, though it does shrink some... probably not a prob for laminating.

    A solution from a GitRDone mentor: sheet rock screws through, then break them flush with vice-grips, once the glue's set. We were skeptical, but it went fast and we never had rust spots through (latex) paint. Still, those Raptors sound like the bomb.

    I note that you have a LOT of longitudinal nailers. I'm guessing your pattern can be a little less, relying on them to do the heavy work.

    Good luck, however you go!

    Dave Z

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  4. The chest-prod sketch-up pic did make me laugh.

    Good luck and following with interest!

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  5. I'll look into Gorilla Glue as well then. It is about a buck an ounce here in Canada on Amazon... that's for the 33oz bottles. The building supply places only have it in the small bottles at a crazy price. Dave, do you know how much I would need - per square foot of glue joint say? And thanks everyone!

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  6. Here's a link to a scientifically conducted glue test:

    http://www.oldbrownglue.com/pdf/HowStrongisYourGlue_FWW.pdf

    It isn't the first test I've encountered that has had similar results. In most cases Titebond 3 is superior to Gorilla Glue in all respects. Also, the posts I've read in the woodworking forums generally favour Titebond 3 over Gorilla Glue. The main thing about Gorilla Glue with a gap or poor fit - so I've read - is that it will expand to fill the gap, but the foam has no strength whatsoever.

    Also, Gorilla Glue is as expensive as cheaper epoxy up here, and is difficult to get off your hands, so I think that I will not bother with it at all. I do respect the opinions and experience you guys have provided though and I certainly don't want you to stop :-)

    So for me, it is between Titebond 3 and epoxy. I was able to source 4 gallon jugs of T3 for $153.00 taxes in and can get more at that price so for me it makes it well worth it to do everything to make it work. Whatever I can save money-wise to do my laminating will well offset the cost of the epoxy and Dynel I need to sheath the bottom. That will be 2 grand or more itself, based on the amount of epoxy resin that gets soaked up by the Dynel (giving the abrasion resistance). So I think I will do the test in the post and see how it goes.

    And Dave, I WILL pad my chest prod, despite my wife's recommendation that I attach it to my head. Harder, she says.

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  7. The view from the cheap seats: roofing tar, screws with ends busted off by vice grips, clean up the exterior well, then Chris Morejohns liberal coating with fiberglass, matte, and cloth. NOT epoxy, at all. Oafish, brutish, simple, way cheap. Pass the jug, por favor...........

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  8. Can't quite go there yet Robert :-) I did make up my second test sample today and will cut it up for analysis tomorrow. Two things I noticed: It will take a lot of glue. My 3 by 3 sample used up about one fifth of a gallon jug, spread with a paint brush to the thickness and amount I felt comfortable with, and I can say with relief I don't need a chest prod. Knee pressure works just fine :-)

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  9. I'm following your project with great interest. I am also retired although I have nothing like your experience or resources.

    When completed do you intend to sell plans?

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  10. Thanks for the comment Everitt! I'm documenting as much as possible and will soon begin to include videos. I don't think that I would produce formal plans but a book is a possibility given a successful project. My wife and I plan to continue blogging after launch as well. Whatever we sell, if at all will be at modest cost. I'm hoping that as blogs become gain greater following I can generate some supplemental income from advertising. But income or not it's 'shits and giggles' anyway as they say :-)

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