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Filling Knots with Epoxy

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Some of the most amazing pieces of wood are the ones that have “defects.” Defects could be spalting (mold), small checks, bark inclusions or, in this case, a huge hole. Sure, we could cut around those things and eliminate them, but I’m a huge fan of including this character in the final piece. I think it helps make my projects unique, and capitalizes on the natural beauty of wood. So, let’s look at filling knots with epoxy.

The Resin

The product that will give us the best results for filling knots with epoxy is casting resin. This is very different from the two-part epoxy in a syringe you’d buy at a home center or hardware store. It flows better, and it cures more slowly, so you have more working time. Be sure to carefully follow the manufacturer’s directions for mixing. You’ll probably have bubbles in the resin after you pour it. No problem, we show you how to get rid of those.

Then what?

Once the resin is poured, give it ample time to cure. It cures much more slowly than off-the-shelf epoxy. Once it’s completely cured you can sand it flush with surrounding wood, and admire your work.

Other Uses For Epoxy

In addition to filling knots with epoxy you can use it to fill or bridge other defects. Lots of woodworkers are commonly using epoxy for wood cracks. Again, instead of cutting the defects out, we can make them an interesting part of the finished piece.

General Repairs

If, instead of highlighting defects like cracks, you want them to go away, check out our advice on how to repair wood cracks.

Discussion
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25 Responses to “Filling Knots with Epoxy”
  1. Mike

    When you are waiting for all the bubbles to rise to the surface, will vibrating the wood help them to come up faster, or will that hinder the process?

    Reply
    • Customer Service

      Hi Mike. I don’t think agitating the wood would help bring the bubbles to the surface. It takes a while for all the bubbles to come up. You just have to be patient, let them surface, and hit them with heat when they do.
      Thanks
      Paul-WWGOA

      Reply
    • Regina Campuzano

      Smooth vibration does make all the bubbles come out. I used it for doing dummies, works great. Also mixing delicately with a round stick.

      Reply
  2. ROBERT

    Nice tech George, have you noticed yellowing over time with this? I have used several two part mixes, and often after several years I see discoloration. Also, were you able to flatten that board?

    Reply
    • Customer Service

      Hi Robert. I’ve got epoxy patches that are about two years old, with no yellowing. So far, so good…..
      No, I haven’t done any more work with that piece of walnut yet. It’s killing me. I can’t wait to see that amazing board under finish. But there are other priorities.
      Thanks
      George-WWGOA

      Reply
  3. Jestin

    It is my understanding that it’s not heat that takes out the bubbles, simply exhaling on them also does it. I believe it’s the carbon dioxide that removes the bubbles.

    Reply
  4. deborareda

    Great instruction video. However, I am surprised you didn’t wear a dust mask or gloves. In my experience the dust, even from casting resin can cause irritation to your skin and thogh there aren’t any noticeable fumes, the fine dust more than likely isn’t healthy to breath. ( also regarding gloves, I always manage to get some on myself, which requires acetone to clean up. 🙂 ). I also use crushed stone or shells to fill large voids.

    Reply
  5. Tony

    After filling the voids with the casting resin and letting it cure, can you run the board through a board thickness planer?

    Reply
    • Customer Service

      Dear Tony,

      Thank you for your patience. In response to your question-

      I have seen opinions on both sides of this and haven’t tried it myself. I’d be concerned about the heat build-up causing problems softening up the epoxy and gumming up my planer blades, so wouldn’t be inclined to try it.

      Paul
      Woodworkers Guild of America

      Reply
  6. anthonybyrde

    Very interesting and useful class, thanks. I use a set of digital postage scales that measure in grams or decimals of an ounce to measure a very precise 50 – 50 mix (or whatever the resin requires). I then stir for at least two minutes and including scouring around the edge of the pot. That will ensure a correct cure and should help to prevent discoloration, which can be caused by either over-heating or an excess of one of the resins. If it is a slow cure resin/hardener combination you can leave it a while for the bubbles to come out before applying it to the cracks, preferably in a shallow tray to help avoid heat build-up.
    I would also echo the other comments about protective clothing: we all think epoxy is safer than polyester resin because it doesn’t smell as bad, but it can be toxic. For sanding I use gloves, fine-grade dust-mask, and paper overalls. You also don’t want to have to get the resin off your skin with solvent, either. Anyway a great session.

    Reply
  7. John

    Great video, Mike. What types of dye should I use if I want to add some color to the epoxy?, If there are still voids in the board after the epoxy sets up can I simply pour on more epoxy and wait another day for it to set or will I need to rough it up somehow to get it to bond? Also, are there any issues with running the boards containing epoxy fill through my planer?

    Reply
    • Customer Service

      Dear John,

      Thank you for your patience. In response to your question-

      Here is the dye that George recommends: https://amzn.to/2M87ltH

      I would abrade the epoxy with 150 – 180 grit sandpaper before pouring another layer for better bonding.
      As far as running epoxy through a planer, I have seen opinions on both sides of this and haven’t tried it myself. I’d be concerned about the heat build-up causing problems softening up the epoxy and gumming up my planer blades, so wouldn’t be inclined to try it.

      Paul
      Woodworkers Guild of America

      Reply
      • John Whiting

        I’ll try it. Keep you informed on my experience. Thanks for your opinion however.

        Reply
  8. John Whiting

    Hey George, How bout running it through the DeWalt Thickness Planner? After plenty of cure time!

    Reply
  9. walt

    whoa! do you not have lung cancer in the States? i really dont think its a good idea to encourage people to sand epoxy [or some woods’ for that matter] without first instructing them of the potential health risks of inhaling this fine dust. i work with epoxy all the time and would never-ever sand without an efficient extraction system & wearing a fine filter mask. In fact, if there is still evidence of dust on cloths or hair, climb into the shower and wet it down – then take off the mask!
    You point out the dust on your spectacles, so imagine what you were pulling in to your lungs with every breath whilst sanding this blank – not a wise course with epoxy dust.

    In my professional opinion you should remove this video (which is otherwise very instructional) pdq and replace it with a healthier version.

    Further: ‘guess-gauging’ epoxy mix quantities is also very bad practice & to be honest just plain lazy. For best results the manufacturer gives precise ratio’s for a reason; it isnt guess work and can be dangerous [if you’ve ever seen a student over do the catalyst causing the pot, first to boil, and then ignite, it isnt a joke i can tell you!]
    Work-safe, Stay-safe 🙂
    walt, wales uk

    Reply
    • Customer Service

      Hi Walt. I appreciate your feedback and it has been forwarded to the proper department. Your comments are important to us and help with the development of our online video streaming community.
      Thanks
      Jean-Woodworkers Guild of America Video Membership

      Reply
    • Customer Service

      Hi Michael. I have never done this, but as long as it is cured I would think it would work. I’d suggest doing a small pour on some scrap to test. You might have to experiment with your router speed to get optimal results.
      Thanks
      Paul-Woodworkers Guild of America

      Reply
  10. Georgia Davison

    Considering how much dust was on your glasses and shirt, sanding WALNUT and EPOXY, meant you SHOULD have worn a DUST MASK!!!

    Reply
    • Customer Service

      Hi Georgia. I appreciate your feedback and it has been forwarded to the proper department. Your comments are important to us and help with the development of our online video streaming community.
      Thanks
      Jean-Woodworkers Guild of America Video Membership

      Reply
  11. Regina Campuzano

    Smooth vibration does make all the bubbles come out. I used it for doing dummies, works great. Also mixing delicately with a round stick.

    Reply

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