Filling Knots with Epoxy
George VondriskaDescription
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.
I picked up this beautiful piece of walnut recently. I just couldn't not buy it. I don't know what I'm gonna use it for yet. Some kind of a tabletop. But, one of the problems it has is that bark is just a little bit loose. And I want to make sure that that bark stays in place because that's such a nice feature of it. The other thing is, it's got some knot holes. It's got some cracks and I want to take care of those. I want, I don't want to just leave them as cracks. So we're gonna fix this. We're gonna take care of this with epoxy, more specifically, this is what's called casting resin. If you go to a hardware store, if you go to a home center, and you get two part epoxy in the syringe, you squeeze it out, you mix it. It's really thick compared to this stuff. You'll see that in just a second. With this, benefit is I can pour it over the top of this and it's really gonna flow down into that void. And it's gonna do a much better job on this than trying to pack the regular two part epoxy in there. So, casting resin, it's available online from a variety of sources. Follow the manufacturer's directions about mixing it, There are some pretty persnickety directions here about what you do to get it mixed. Now, this goes all the way through when I pour the resin, I don't want it to go all the way through. So we're gonna build a dam in the form of masking tape. And what I'm doing here is, I'm choosing my good face-bad face. Actually my kid Jenny, chose good face-bad face. So this, by putting the masking tape on this side is becoming the bottom of this board. All right, a little mixing. And this stuff, when I pour the hardener in here, you'll see, it's got a little bit of a yellow tone to it, but it doesn't when it's cured. When it's cured it'll be perfectly clear. 50/50 mix. The other thing that's different between what I'm working with here, this casting resin and a syringe of most two part epoxy is that the cure time, the tack time is really long. This is gonna sit overnight to be completely dry. So as a result, we don't have to be in quite a foot race here with the glue in order to make this work, where if you were working with five minute epoxy right now, you'd have to be working really, really, really, really fast. All right, here's the easy part. I'm just gonna pour and let it settle down in there. We're gonna fill that void right up. And this is where the fact that it'll flow on its own is a good thing. So one of the things you probably noticed is there a lot of bubbles in there, and that is a natural by-product of course, of stirring this epoxy around. We're gonna fix that. And the form of fixed, I'm gonna do it with a propane torch but you can also do it with a hairdryer or a heat gun. We need heat is what makes this work. What I'll do is I'll light this up and then pass it over the top, watch closely. It's gonna just take all the bubbles right out of there. This is gonna be so beautiful when it's done. Now here's a qualifier. On a pour, especially where all this bark is right here, watch that, see that bubble just come up? Watch this over time, because it's gonna continue to bubble, that bark is a really rough, irregular surface. So we've got air trapped down in there and you just want to come back and look every few minutes. If there are new bubbles, hit it with heat again. New bubbles, hit it with heat again. Eventually as this stuff cures that'll stop because the bubbles can't come up anymore or all the bubbles that are gonna come up will have come up. So look at it every once in a while. We're gonna let this cure. And then once it's dry, come back and look at leveling the surface out so that we get rid of the excess epoxy. And just what that's gonna look like. When you're using epoxy or casting resin to do this kind of stuff, do make sure you let it dry, dry, dry, dry, dry. This stuff has actually been sitting almost two days since I cast it. One of the things that's really nice to do is to keep the mixing pot. And there'll be a little bit of epoxy left in that. And you can watch that to see how hard it's getting. 'Cause of course, we don't know what's happening down inside here, layers and layers and layers below the surface we can touch. So best bet if you're not sure, give it more cure time. Does take a while to completely cure out. Now, we've got epoxy that's standing proud on the surface. This is where we have to introduce the S word. We have to do a little bit of sanding. I've also used cabinet scrapers on this and it works pretty well. Cabinet scrapers work pretty well. Downside to sanding the epoxy, you do tend to go through abrasive fairly quickly 'cause the epoxy clogs that just a little bit. So I'm gonna start with a belt sander. Then we'll move to a random orbit sander and we'll just kind of talk through the steps it takes to get this look and really good. You can really start to see the dust on my glasses. You can really start to see the game that we're getting from this epoxy, because what we had here before was a bunch of bark that was really ready to fall right out of the slab. Now it's been stabilized with the epoxy. What needs to happen is this is 120 grit on the random orbit right now. I'll keep taking that up there, progressively finer grits. We want to sand that up to about 400 or so, then when we get finished over the top of this, that epoxy is gonna really look great inside that bark inclusion. So epoxy, casting resin is a great way to save a beautiful board like this that otherwise would have a huge hole in it and wouldn't look that good. Give it a try the next time you run into these kinds of inclusions in the wood for your projects.
The ad for casting resin says it dries white. When filling a knot hole, wouldn't you want it to dry clear...so when you sand it flush, you can see the knot/imperfection through the resin? I'm confused by that description of it drying white????
<strong> Ticket 36955</strong> In the epoxy Woodworking class they mentioned using penetrating epoxy to avoid leaching. George didn't use it in this video. Is it necessary or does the casting resign just not leach?
George, I watched your video and since I want you to live longer. I wanted to recommend that you wear a mask when sanding. The dust on your glasses is a definite indicator of the dust your breathing. As you are aware, that's "No Bueno". Take care and as always, I love your knowledge and teaching skills.
Great overview, thanks! !!! BUT !!! Use an appropriate respirator...inhaling saw dust and finely ground epoxy is very bad for you.
Smooth vibration does make all the bubbles come out. I used it for doing dummies, works great. Also mixing delicately with a round stick.
Considering how much dust was on your glasses and shirt, sanding WALNUT and EPOXY, meant you SHOULD have worn a DUST MASK!!!
Hi , before sanding the resin would running a planing router over it be advisable?
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
Hey George, How bout running it through the DeWalt Thickness Planner? After plenty of cure time!
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?