Using West System epoxy to fill deep voids

[Royce Meritt]

For the past couple of years I have been using some Burr Oak from a huge tree where my wife grew up. I had the tree milled and kiln dried. It isn't the greatest material due to knots and other imperfections but on the right project I like the "rustic" look of the wood. Over time the pile has become pretty well picked over and I am left with the pieces with a lit of larger knots, voids and other imperfections. In the past I have used a mixture of wood glue and sanding dust to fill minor imperfections, mixing up a batch about the consistancy of peanut butter. So, on to my question...

This time, because of some larger imperfections I would like to use West System epoxy to make my "wood putty" due to the larger voids, knots, etc. To what consistancy should I mix sanding dust into the West System epoxy so that I still get a good color match but not lose the ability for the epoxy to "flow" into the voids? Any other thoughts? Thanks.

Hope that all made sense!

[John Downey]

I don't think it's going to flow well at all with sawdust mixed in. We used to dye it black for mesquite, emphasize it rather than try to hide it - Proline universal dye goes right in (you can get universal dyes at real paint stores rather than hardware or home centers).

Another thing I've seen done is to fit in another chunk of wood, trying to look like a knot or branch but filling the really big void. Never much cared for that route myself, even good ones stuck out as "fake" to my eye.

[Jack Battersby]

Hello Royce, what you are suggesting is a very (very ) common practice in boat building. In fact, we order our sifted wood flour in 50 lb bags, and you would be amazed at just how much sawdust you need to get to 50 lbs. I am not a huge fan of West System as a brand, but these techniques work with any laminating epoxy. Do not use the fast setting hardener unless you are working in a cold environment and get it out of the cup within the first few minutes or it will start to harden up on you. Don't use instant hardener unless you really know what you are doing and can work very fast. Remember that epoxy is heat activated and when the cup starts to warm, you are done using it. Using wood flour as a filler will lessen the pot life of the epoxy.

Before I go too much in depth here, I have a video up on my web site called "Using Epoxy as a Glue" you can view it at http://www.sandypointboatworks.com/wood-boat-school/video-library

The only difference in using epoxy as a glue and using epoxy as a filler is the amount of filler you put into the epoxy and the types of fillers that you use. As you are trying to stay natural, the only filler you need to be concerned with is wood flour as all the others need to be painted.

In general, all wood flour will darken up significantly when added to epoxy. You do have some control over hue of the final color with different types of wood flour, but that is fairly limited.

As for your concern over the "flow" of the epoxy, it is a non issue. There is absolutely no need to mix the filler into a putty consistency, just mix in enough wood flour until it runs off the mixing stick with a runny consistency that you are happy with and you are done. If you mix it to the consistency of a putty, peanut butter or maple syrup it will make no difference in the final color or the fill.

When building a boat, whether we are filling a crack, making a fillet, Gluing a gunwale or easing the edges on an inside corner to put fiberglass on, the only difference in the mixture is the amount of wood flour in the mixture and how much we want it or don't want it as the case may be to run. Be sure and use sifted saw dust and get out all the big chunks. The finer the better, there is a reason why they call it wood "Flour". a fine belt on a stationary sander will do the trick.

Keep in mind that epoxy sands much harder than wood filler and once you sand it, it will turn milky when sanded. Don't be concerned about this as it will brighten right up with a top coat of varnish or another coat of clear epoxy. Also remember that if you are using this outside, that Epoxy needs a UV protection layer.

When I build furniture, I use epoxy and microballoons as a fairling tool on joints which will be painted for a perfect, shrink proof surface to paint.

Good luck.

[Royce Meritt]

Thanks for the thoughts, John. I have seen the epoxy/black dye you mentioned and it can look really nice. In this case I want to make the "repair" look a bit more natural.


I don't think it's going to flow well at all with sawdust mixed in. We used to dye it black for mesquite, emphasize it rather than try to hide it - Proline universal dye goes right in (you can get universal dyes at real paint stores rather than hardware or home centers).

Another thing I've seen done is to fit in another chunk of wood, trying to look like a knot or branch but filling the really big void. Never much cared for that route myself, even good ones stuck out as "fake" to my eye.

[Royce Meritt]

Hello Royce, what you are suggesting is a very (very ) common practice in boat building. In fact, we order our sifted wood flour in 50 lb bags, and you would be amazed at just how much sawdust you need to get to 50 lbs. I am not a huge fan of West System as a brand, but these techniques work with any laminating epoxy. Do not use the fast setting hardener unless you are working in a cold environment and get it out of the cup within the first few minutes or it will start to harden up on you. Don't use instant hardener unless you really know what you are doing and can work very fast. Remember that epoxy is heat activated and when the cup starts to warm, you are done using it. Using wood flour as a filler will lessen the pot life of the epoxy.

Before I go too much in depth here, I have a video up on my web site called "Using Epoxy as a Glue" you can view it at http://www.sandypointboatworks.com/wood-boat-school/video-library

The only difference in using epoxy as a glue and using epoxy as a filler is the amount of filler you put into the epoxy and the types of fillers that you use. As you are trying to stay natural, the only filler you need to be concerned with is wood flour as all the others need to be painted.

In general, all wood flour will darken up significantly when added to epoxy. You do have some control over hue of the final color with different types of wood flour, but that is fairly limited.

As for your concern over the "flow" of the epoxy, it is a non issue. There is absolutely no need to mix the filler into a putty consistency, just mix in enough wood flour until it runs off the mixing stick with a runny consistency that you are happy with and you are done. If you mix it to the consistency of a putty, peanut butter or maple syrup it will make no difference in the final color or the fill.

When building a boat, whether we are filling a crack, making a fillet, Gluing a gunwale or easing the edges on an inside corner to put fiberglass on, the only difference in the mixture is the amount of wood flour in the mixture and how much we want it or don't want it as the case may be to run. Be sure and use sifted saw dust and get out all the big chunks. The finer the better, there is a reason why they call it wood "Flour". a fine belt on a stationary sander will do the trick.

Keep in mind that epoxy sands much harder than wood filler and once you sand it, it will turn milky when sanded. Don't be concerned about this as it will brighten right up with a top coat of varnish or another coat of clear epoxy. Also remember that if you are using this outside, that Epoxy needs a UV protection layer.

When I build furniture, I use epoxy and microballoons as a fairling tool on joints which will be painted for a perfect, shrink proof surface to paint.

Good luck.

Thanks for the input, Jack. You make some very good suggestions. I think I'll give it a go. I do have the slow-setting hardener. I'll mix up small batches with just enough wood flour to give me the color I'm looking for but still allow some "flow". I'm not too worried about the fact that the wood flour will darken. This Burr Oak has a very "rustic" look to it so the color of the wood is all over the board. Thanks again.

[Sam Murdoch]

One potential problem when filling voids with epoxy is bubbles. Air bubbles will highlight a fill and utterly defeat your intention. Ask me how I know. :)
Pour slowly after you have stirred carefully.

[Prashun Patel]

For large voids, mixing with dust can cause the result to look mealy. If it were me, I'd fill it with untinted epoxy which will look like amber glass once finished.

I would also fill in stages.

[Jack Battersby]

I think Royce will have to mix up a few small batches and just do a little experimentation to see what he likes. Using unthickened epoxy in large voids is not advisable when whatever you are building will be flexing on a regular basis on a regular basis as cured epoxy with no filler or fiber is a brittle substance. If it is being used on furniture though it wouldn't be an issue.

[Art Mann]

Thanks for the input, Jack. You make some very good suggestions. I think I'll give it a go. I do have the slow-setting hardener. I'll mix up small batches with just enough wood flour to give me the color I'm looking for but still allow some "flow". I'm not too worried about the fact that the wood flour will darken. This Burr Oak has a very "rustic" look to it so the color of the wood is all over the board. Thanks again.

I just want to endorse Jack's information. I also come from a Boat building perspective and my experience with epoxy is exactly the same as Jack's. Pay special attention to his comment that epoxy is much harder to sand than wood filler or the surrounding wood. This property makes the technique of applying the material proud to the surrounding surface and then sanding flush difficult.

[Ryan Mooney]

I wish I'd read Jacks information before my last attempts at doing this, would have saved me several batches of epoxy and a bit of a learning curve! Most excellent summary.

One additional thing I found was if you do the fill in two steps with a slightly thicker mix for the first pass and then a thinner mix on top it was easier to contain the "run through" you get if its to thin and the thinner top coat made it easier to level the top which was harder when it was to thick. I somewhat compensated accidentally for the sanding variance by skim coating the area around the fill with a thin shot of epoxy and once I hit wood it was "level enough".

[Jim Andrew]

To fill knots that fall out, I turn tapered pieces from the same kind of wood and fit them and glue them in.

[John Piwaron]

For large voids, mixing with dust can cause the result to look mealy. If it were me, I'd fill it with untinted epoxy which will look like amber glass once finished.

I would also fill in stages.

I've done that with mesquite. In fact, it's my kitchen table. One area in particular had a huge void. I filled it with West System. No fillers at all. It took 5 or 6 pours to fill 'er up. Yes, it's a translucent yellow in that spot. But overall, all the fills anywhere on the table look black. There's no where for light to get in and reveal the unfilled epoxy. So it's black. Except that spot.

[Jack Battersby]

The yellowing that people are talking about is mostly a function of the manufacturer. The better the grade of hardener, the less yellowing you get. MAS and RAKA yellow much less and in general than West System, the slower hardeners are also less amber than the faster hardeners. Of course none of that really matters unless you are clear coating or using unthickened epoxy.

Also, if you are going to pour in stages,which is only really necessary if the the gap is very large, then be sure that you are either using a non-blushing epoxy, or be sure and let it cure between layers and clean up that waxy build up. Most of the newer formulas have a non-blushing hardener, not sure why anyone would want to get an epoxy that blushes but West Systems does still sell it unless you buy their special non-blushing hardener.

[Malcolm Schweizer]

West 207 special clear is (as the name suggests) clear and also has a UV inhibitor. RAKA sells a UV epoxy but it has a very blue tint. Any pour of 1/2" or more will look very blue if left as-is.

[Jack Battersby]

We go through about 150 gallons of RAKA a year and they have been trying to get us to switch to the UV inhibited formula since it came out. I did test it and we pretty much only use it for laminating cloth, fillets and other boat related tasks so in our shop, a 1/2" pour of clear epoxy is pretty much unheard of unless it was left in the mixing cup from a job.

I look at it like this. UV inhibited, IS NOT UV protected and therefore I am going to protect it with Spar Urethane anyway, so what is the point.

I still see some of our boats that are 30+ years old that still look like they day they left the shop. Some where built with West, some with System 3, some with MAS and some with RAKA. All of them get a little love every few years (or less depending..), with a fresh coat of Spar Urethane.

I will probably not switch until someone comes out with a UV protected formula, and I am sure they will sooner or later. And I am most certain that will likely still put Spar over the top of that.

[Stephen Cherry]

One thing I have tried is to add a little white pigment in to a sawdust epoxy mixture. As noted above, the epoxy darkens up pretty quick, and the white lets you modulate it.

Of course, all of this will take some experimentation.

[Bob Michaels]

West System advises to "wet out" the cavity with un-thickened epoxy just before putting in the thickened epoxy. Thickened in this sense means the epoxy mixed with sawdust or other thickeners. I've used this method many times and it seems to make sense and do a good job.