Scarf Joint

Discussion in 'Wooden Boat Building and Restoration' started by sleepyweasel, Sep 17, 2012.

  1. upchurchmr
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    upchurchmr Senior Member

    West System says thinning epoxy will ruin the strength. The regular west will wick into the wood as far as it is going to. Some people think the epoxy will penetrate farther with thinning, but that does not happen. Even if it did and the strength was much less it won't help.

    If the joint breaks in the wood you cannot do better.

    Thinning is a bad idea for epoxy carried over from paints and possibly other glues.

    Best way to decide is to make the same joint both ways and break it. Then cut another sample and use a glass to see how far the epoxy has penetrated. That way you don't have to believe somebody you don't know.
     
  2. pauloman
    Joined: Jun 2010
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    pauloman Epoxy Vendor

    I found a belt sander works best. The plys form nice stripes that tell you how evenly you are sanding.... Note that when gluing with epoxy, don't clamp too tightly or you squeeze out the epoxy and starve the joint, resulting in a failed bond. I will usually cover the joint with a piece of plastic and then set on cinder block on top....

    paul
    progressive epoxy
     
  3. sleepyweasel
    Joined: Aug 2012
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    sleepyweasel Junior Member

    Thanks for all the answers. They helped out a lot.
    What I done was the practice joint and it went together so well I began working on the sides and bottom of the boat. The power plane done a great job as long as I went with the grain, but when I tried one piece 90 degrees to the grain I got the splintering on the edges. I also found that a good solid base under the piece that you're scarfing seems to help a lot.
    I used wood flour from West Systems. They call it something else, but I know it's wood flour. Very fine, wood smell, and creates a nice texture in the epoxy. Takes a lot of the stuff to thicken it however.
    My floor went together pretty good, but my sides I didn't have such good luck with. Not too good at clamping things together I guess, and ended up having to re-glue them. One I stuck a saw blade in and roughed it up to give the epoxy something to key into and the other I used a big hypodermic to shoot thickened epoxy into. Then I clamped them very well and they look good.
    I think the scarf is very strong and have no fear of them giving way. In fact, I think the plywood would break before the joints would give out.
    Thanks again, really appreciate the help.
    Daniel
     
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  4. troy2000
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    troy2000 Senior Member

    Sounds like you understand a basic truism of boat building (or any other enterprise): instead of blindly following a rigid set of instructions that say "do this, then that, then the other....," you need to go with the flow and deal with what's in front of you.

    I have no doubt I'm speaking for a lot of other folks when I say we're looking forward to updates on your project. And pictures, of course. Lots of pictures.....
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Thinning epoxy does progressively decrease it's physical properties (including strength). If you have a clue about the affects dilution rates and their relationship with the physical properties, then you can thin epoxy, of course knowing what will happen. I know old school builders that dilute epoxy over 50% and make absurd claims about it's effectiveness. They're just talking out their butts, as no tests can confirm these types of dilution rates. The best method to thin epoxy is with non-reactive viscosity modifiers. Of course, working with these requires even more understanding or the molecule you're screw with. This is why my recommendation for the back yard guy, looking to build or repair something is to not screw with things they don't understand and use accepted, proven methods, including viscosity reduction techniques.

    Daniel, West 405 is what you probably used. It's wood colored (sort of) and a small about of wood flour is present, but mostly for color. It's more silica and cotton flock, than wood flour and I'm pretty sure they use some talc too, smoothing out the mixture, though I can't get Tom over and West, to fully admit to some of their secrets, in the filler formulations.
     
  6. yellowcat
    Joined: Feb 2007
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    yellowcat Junior Member

    Have you worked with both Meranti and Okoume WBP in both cases ? Noah's marine supply offer both. Meranti is stronger and stiffer it will allow less penetration of epoxy ? There is quite a weight differential, 3 x 3 mm is pretty much the minimum i am willing to do for a hull, if Meranti means it is like 4 x 3 mm Okoume ... But for tortured ply with complex curves Okoume might be the only way. I am considering a mixture of both.
     
  7. PAR
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    PAR Yacht Designer/Builder

    No, I've never used either species of plywood . . .
     
  8. sleepyweasel
    Joined: Aug 2012
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    sleepyweasel Junior Member

    Yeah, I believe it is 405 that I have been using to thicken the West System epoxy. I also have some silica based thickener that I'm planning on trying for fillets when I begin the stitch and glue process on the hull.
    Kinda off subject, but is it adviseable to wet out the joints with un-thickened epoxy before putting in fillets? Seems reasonable, as if water got into the unprotected wood it would begin rotting, but I'm wondering if it is necessary
    Thanks, Daniel
     
  9. yellowcat
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    yellowcat Junior Member

    If it is under the water line or 6 inches near (above) the water line, i always make certain that i have fully impregnated any wood to the best i can depending on the quantities needed, you can use means to force it into cracks. This is the best area for condensation when the water is cold particularily . I did not on my Paceship sail boat and i had to play dentist in some areas. And above the water line, it is better . Last time i used silica, it was a mess, we had to work fast and epoxy gelled rapidly, we had to do it in phases, it was about 85F outside.
     
  10. yellowcat
    Joined: Feb 2007
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    yellowcat Junior Member

    if you want to see my Paceship, check http://auxbergesdespins.wordpress.com/
    it has a nice line, people take pictures especially with my new (5 years ago) paint job, done quickly mind you. This is a 1966. I used to put it on my mooring in the middle of the bay, it was picturesque indeed, when the sun hits this yellow color, it is so bright.
    Needs love each year ...
     
  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Dan, log onto westsystem.com and systemthree.com and download their user's guides. These will nurse you through most of the basics. I would take Yellowcat's advise with a grain of salt, consider his last few, flawed procedures offerings. This isn't a personal dig at Yellowcat, just an observation that some of his suggestions are against established practices.

    To directly answer your questions, it's always advisable to wet out raw wood, before applying a fillet, to prevent joint starvation. If the surface has had prior coatings of epoxy (at least two) then you just need to "tooth" up the area, before applying the fillet.

    Most fillets are a combination of fillers. Silica is used to control viscosity, with other materials add to the physical attributes of the epoxy. No single material works well for everything, unless it's a pre-mix, like West 405 is. In fact 405 is designed for making strong fillets.
     
  12. Ilan Voyager
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    Ilan Voyager Senior Member

    Meranti glues very well, no worries. For compounded ply go to okoumé as it bends very well. But I've used meranti for topsides without problem on various projects using a method based on the use of strip planks bottoms et compounded ply at the topsides. I f are using a mixture, better keep the okoume for the compounded pieces and meranti for the flat ones.

    Better pre-wet. Silica is a "geller" so it doesn't release resin for the wood. My personal experience is to use as litlle silica as possible just for controlling the thixotropy. Silica make rather brittle joints and it's a pain to sand. My preferred filler is maple flour (but it's very dark) with a few % of silica, cheap, creamy, easy to sand and very strong. Works on 90% of the situations.

    On scarfs a lot of grain wood is exposed and it's a very thirsty thing. So prewet always, twice better than once until saturation. And let cure...Most of the problems with plywood scarfs come from insufficient curing and or not enough wetting.

    For making straight scarfs on plywood the best is a circular saw with a scarffer http://www.jamestowndistributors.com/userportal/show_product.do?pid=4378 pretty expensive for 2 pieces of steel but having a scarffer and good table makes the job a piece of cake. If you have dozens of scarfs to make it pays by itself. Also saw blades are cheaper that planer blades; the glue lines kill them fastly...On 3mm ply for a cylinder mold panel a belt sander with guide is largely enough.

    For me electric planers and routers a the more dangerous and unconvenient tools for making scarfs.
     
  13. pauloman
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    pauloman Epoxy Vendor

    When gluing with epoxy, care has to be taken not to clamp too tightly and squeeze out all the epoxy. That said, I recently had a customer pass on a really good 'trick'.

    Ground walnut shell is a common anti slip additive to marine epoxies and other coatings (because you can easily sand it off in the future). This person adds some of the ground walnut into the epoxy used for gluing. This way he can clamp tightly with the walnut shell grit providing enough spacing that the epoxy cannot be completely squeezed out. Clever!
     
  14. upchurchmr
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    upchurchmr Senior Member

    Pauloman,

    A Plastic mesh bag or sleeve can work well also, needs to be very open, the mesh strands need to be about .005 to .010 thick, just place it on the surface to be epoxied and the joint cannot go fully closed, always a controlled amount of epoxy.
     

  15. LP
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    LP Flying Boatman

    Makes me think fibreglass dry wall tape would work also.
     
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