epoxy fillers used for plywood joints

Discussion in 'Materials' started by latestarter, Sep 6, 2012.

  1. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    I have been reading up on stitch and glue methods for plywood boats and the authors often say add fillers to strengthen the epoxy.

    The customer data sheet that came with the epoxy gave its properties as
    Tensile stress 8,800 psi
    Flexural strength 14,900 psi
    Compressive strength 12,300 psi.

    These are well in excess of anything plywood can achieve.
    The accepted wisdom is that the epoxy joints are stronger than the wood.

    Obviously fillers are needed for anti-sag or stopping epoxy draining out of glued joints and colouring the epoxy to match the ply.

    Given that the plywood is the weak link in the chain, has strengthening the epoxy any benefit?
  2. tom28571
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    tom28571 Senior Member

    Look carefully at the context that the authors are referring to. Some builders only add a thixotropic material like cab o sil to glue in joints while some add wood flour. Where more strength in the epoxy is needed, glass microfibers are often used. For filets with glass tape, many use wood flour or sawdust to bulk and extend the epoxy.

    Failures will rarely be due to strength of the materials but rather to improper application.
  3. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    This is the point I was looking for advice on, where on a plywood boat would it help. I can see if working with stronger materials like fibreglass it would be beneficial/essential.

    Good last point, I should concentrate on workmanship rather than worrying about what is largely an academic issue. A dry joint is useless however strong the material used.
  4. PAR
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    PAR Yacht Designer/Builder

    From a technical stand point fillers decrease straight resin's physical properties over all, though some aspects may be improved. Point being that resin alone tends to be brittle, so improvements to elongation, peel and tension strength, compressive strength, stiffness, etc., all can be addressed and made application specific.

    In all cases, filler mixtures should be application focused. A fairing compound is quite different than a structural fillet mixture. Substrate considerations also should be addressed.

    Simply put, you'll find you're mixing 3 or 4 different formulations: a heavy structural, a light structural, fairing compound and bonding mixtures. Each of these can be tailored to the substrate to further insure things stay stuck. There are also specialty mixes, such as temperature sensitive, fire protection, surface smoothness, etc. Again filler materials are selected to offer the improvements for the parameters of the task. Graphite for the bottom of a preformance dinghy for example.
  5. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    Many thanks PAR, I was hoping you would reply. As usual, a concise yet comprehensive comment.
  6. VanillaGorilla
    Joined: Sep 2012
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    Location: Orlando, FL

    VanillaGorilla New Member

    Follow up with PAR's threads. Absolute resource of knowledge from him on here as well as many others. Thanks a bunch gentleman!!!

    What I know so far....

    The PB mixture works awesome when "bedding" the stringers to the hull and putting together the smooth transitions that are needed with the fillets. I have been using chopped fibers and cabosil to "goo" it up for my fillets and the strength is there as well as the workability. After many many threads, adding fibers and a thickening agent (cabosil, fumed silica, wood flour, etc...) to the epoxy is the benchmark for "bedding" and helping to reduce hard spots along the hull and stringers.

    I am no boat designer by any means but if you are questioning what type of resin to use.....no brainer (EPOXY!!!!!) A few extra bucks is well worth it out there ladies and gents....there is a reason why you buy home supplies at a home store (they are for your home). Leave the cheap PL, hardwood glues, and low-grade polyester to the contractors and think like a boat builder.

    Thanks again to everyone on here who contributes....pretty kick *** indeed!

  7. pauloman
    Joined: Jun 2010
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    pauloman Epoxy Vendor

    this is an example of over-thinking an issue. Any thickened epoxy will be more than acceptable. It takes a lot of thickener to turn epoxy into a paste so for big jobs probably cheaper to purchase a pre-thickened epoxy paste.

    personal plug - I recommend my wet dry 700 epoxy paste - $100 a gallon. Filled with kevlar pulp. can be applied underwater. Last season it was used to repair/rebuld a 100 year old wooden marine railway in 12 feet of water of eastern Long Island NY (saving the shipyard the fishing fleet it serves). mixed it in with 3/4 inch gravel. Has also saved a lot of sinking yachts and floating homes and leaking swimming pools.

    for fairing etc. I have another thickened epoxy that doesn't drag behind the putty knife when you pull the putty knife away...

    still, wood flour, fumed silica etc will both work fine. Note that it takes about 2 part thickener to 1 part epoxy to make a paste.... and that doesn't mean you will get about 3X units of thick epoxy. 2 quarts of fumed silica in 1 quart of mixed epoxy will give you about 1 quart of thick epoxy (and a quart of thickener could cost you $5 -$10 or so).


  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A few years ago I did a price comparison against premixed thickened epoxies and making your own. Other than the convenience of having the premix, which can't be reasonably qualified, you pay a good bit more, in some cases a lot more for the premix. Typically you pay a lot for the materials, when you take percentages and separate prices into account. I guess this amounts to packaging (double cartridges, mixing tips, etc.) and the formulator's ideas of convenience too.

    This said, many not terribly experienced with epoxy may be better off with these premixes as it can take the guess work out of it. Personally, I have formulas that work every time, fairing compounds, glues for different substrates, light and heavy structural mixes, etc. Premixes for me tend to be a little sloppy, in the warm months and don't address substrate differences. It took a while to get a fairing compound down, but now I have two different formulas that are just as smooth and non-draggy as the prefixes.

    The bottom line is familiarity with the various materials. The users guides on the major formulator sites, as well as a lot of good information on Paul's site (> epoxyproducts.com<) can go a long way to answering your questions about what to use and when. Each application will be different, if only slightly. Temperature, viscosity needs (vertical, over head, flat work, etc.), substrate differences, application, etc., all will cause you to modify the mix a bit. On a hot day you may need more thixotropic material to keep it from running out of a joint. Bonding fasteners into wooden holes should have different materiel combinations then bonding fasteners into 'glass or steel or concrete, etc. There are many variables, but again experience will ease concerns as will understanding the information available, like the previously mentioned user guides.
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