Thickened epoxy using sawdust

Discussion in 'Wooden Boat Building and Restoration' started by Raptor88, Mar 19, 2022.

  1. Raptor88
    Joined: Apr 2021
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    Raptor88 Junior Member

    Is there a negative in using sawdust as a filler to thicken epoxy for bonding wood together instead of using a commercially sold filler?

    I'm thinking that using sawdust as the thickening agent, the epoxy bond will still be stronger than the wood being joined and if the joint is pulled apart the wood being joined will fail instead of the epoxy.

    I'm going to build a 12 foot plywood jon boat but never having used epoxy to build a boat before, I'm looking for opinions from experienced boat builders. Thanks.
     
  2. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    I've used wood flour before but not sawdust.
    Minimise your clamping pressures.
    More experienced members will likely post better info.

    What wood are you using?
    And what kind/brand of epoxy?
     
  3. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    The concern I would have is that the sawdust is intrinsically absorptive, so that you could mix a batch to your ideal viscosity, but before cure the individual sawdust particles could absorb enough epoxy to ultimately make a glue starved joint. Cabosil is cheap, and does not absorb epoxy, beyond initial mixing, so you have better control of final outcome.

    J
     
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  4. Blueknarr
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    Blueknarr Senior Member

    Just adding additional concerns.
    Saw dust has inconsistent sizes. It would be difficult to achieve a smooth consistent layer.
     
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  5. Raptor88
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    Raptor88 Junior Member

    Will use West Systems 105 epoxy. Wood frames, sheer clamps and chine logs will be fir. Plywood will be marine plywood though not sure what kind of wood is used.
    Regarding clamping pressures, see my logic in post #7. Thanks.
     
  6. Raptor88
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    Raptor88 Junior Member

    Thanks for sharing your perspective.
     
  7. Raptor88
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    Raptor88 Junior Member

    I will strain the sawdust using a sieve to control the maximum size of the particles. I'm thinking that about 1/16" max size particles will be a good thing to insure that the joints are not epoxy starved regardless of clamping or screwing pressures, since the particles will act as spacers.
     
    Last edited: Mar 19, 2022
  8. jehardiman
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    jehardiman Senior Member

    I have used sawdust filler before and I second this statement. It's hard to get it constant and you end up with a rough surface that requires significant sanding before taping. You can sieve it and grind it to produce wood flour though, which is sold as a filler.
     
  9. cracked_ribs
    Joined: Nov 2018
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    cracked_ribs Senior Member

    Definitely hard to get smooth but depending on the context, fairly fine sawdust works well. It does absorb the epoxy though so you have to watch a bit as you mix... it'll keep stiffening up for a bit after you mix it.

    Wood flour is my favourite filler, actually. Cheap, strong, doesn't really freak me out too much if I inhale a bit of it.
     
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  10. Raptor88
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    Raptor88 Junior Member

    Thanks for sharing your experience. I plan to only use strained sawdust (max 1/16" particles) to thicken epoxy for glue joints. For filleting, I plan to use the West Systems Six10 thickened epoxy that comes in a caulking gun tube. That should negate any rough surface concerns for the fillets.
     
  11. Raptor88
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    Raptor88 Junior Member

    Plan to only use sawdust thickened epoxy for glue joints so there should not be any concerns for smoothness. I'll remove the squeeze outs. Will later fillet the joints using West Systems Six10 thickened epoxy that comes in caulking tubes.
     
  12. messabout
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    messabout Senior Member

    If you have a vacuum accumulator as used on a table saw, use the dust that can be gathered on the vacuum filter. Otherwise buy some real thickener that is more appropriate for the purpose. The cost of the thickener will not break the bank and it will definitely work better than any kind of wood particulate from your workshop.

    As a matter of ill advised haste I have used sawdust for thickener. I will not be doing that again. Buy the right stuff. If you cannot force yourself to use the right stuff, then try talcum powder.
     
  13. Raptor88
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    Raptor88 Junior Member

    Thanks for sharing your perspective.

    It is not the cost of commercial thickeners that I'm concerned about. That cost is minuscule compared to the total cost of the boat and outboard motor. It's that I've seen posts and youtube videos that advise against too much clamping pressure that will squeeze the epoxy out resulting in epoxy starved joints. I'm considering 1/16" max size sawdust particles as the thickening agent so that the particles will act as spacers to insure that joints have enough epoxy regardless of clamp or screw pressures. I will be pre-wetting both sides of the parts to be joined with un-thickened epoxy and waiting until tacky before applying the thickened epoxy.

    Thanks.
     
  14. wet feet
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    wet feet Senior Member

    Maybe its because I live in a damp climate and realise how high the moisture content of my wood is before it becomes sawdust,it doesn't seem like a good thing to introduce a substance that may easily be 16% water to my epoxy mix. I can see that dry wood flour,kept in a sealed container might be a different thing.I bought quite a large tub of microballoons for a relatively small amount and will happily use it in preference to sawdust even though I have a steady supply of sawdust.
     

  15. Blueknarr
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    Blueknarr Senior Member

    1/16" is way way way to thick of a joint.
    Too thick resin is as weak as resin starved joints.

    Many knowable people have told you that it is not a good thing.
    None have endorsed your plan.
     
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