Is Epoxy Good Glue For Wood?
As a Carpenter/Joiner of 30+ years experience I have had first hand experience with epoxy glues, resins, and other treatments and coatings, and their subsequent usage in construction and the marine industry in relation to wood. During my career I have fitted out several live-aboard dive boats and have repaired trawlers that required the fitting or repair/replacement of timber components at times. I feel I am qualified to put an article together to share ideas regarding epoxy glues for wood.
Is Epoxy Glue good for wood?
The short answer is Yes, epoxy glue is good for wood. It’s actually great… however.
The long answer is it really does come under the category of “it depends”.
Epoxy is good wood glue; however, it has limitations as you will discover.
Epoxy shines in marine environments.
Epoxy is exceptional in stable environments.
Epoxy is waterproof.
Epoxy lasts for years.
It is stronger than the timber substrate if applied correctly.
Where epoxy is not the glue of choice is where you would be expecting excessive wood movement.
Epoxy requires an open joint, meaning excessive clamp pressure can cause a glue-starved joint that will fail. If tight joints are required, use other wood glues.
Epoxy resins are too thin to use as glue, they require thickeners added to be effective. Starved joints will occur if not thickened.
Why is epoxy good in marine environments?
Wood has been used in boat and shipbuilding for centuries, and the nature of wood in a marine environment is well known and long studied. Believe it or not, the marine environment suits epoxy glue because the humidity levels are constant and that makes for stable wood.
Epoxy has no flexibility and relies on the substrate, wood in this case, to maintain little to no movement so the glue joint is not placed under undue sheer loads from the timber swelling with moisture and then drying out.
The terminology for this drying then swelling behavior is called cycling, and it is something that occurs in wood and wood glue joints where there are large differences in the relative humidity over time; usually associated with the change of seasons. See image down lower.
In the marine environment epoxy can act as both a wood sealer and a wood glue because of this lack of change in humidity levels. It is always humid so the wood can and does acclimate to this environment and can work with the epoxy glue instead of against it.
Epoxy is waterproof to the extent that it can be subjected to complete immersion for periods of time with little effect on its composition and strength. The question here is what the condition of the substrate is before the immersion took place. Unsealed wood with an epoxy joint will possibly suffer due to excessive wood movement, depending on the size of the piece of wood. Boat transoms are regularly coated with several layers of epoxy before a final finish is applied and this is to stop the wood from swelling, not to protect the epoxy.
When applied in the correct situation and as per the manufacturers directions, epoxy glue will last for years, decades even. It is darned good stuff. To highlight this in real terms, Canada is home to many fantastic woodworkers who make cedar strip canoes out of western red cedar. These canoes are coated with epoxy to protect them from damage and water. Some of these canoes are decades old.
These are just a few examples of why epoxy is good glue for wood.
Now lets talk about where epoxy is not so good.
For this side of the coin we will move inland from the coast a few hundred miles or klms depending where you live.
I will bet London to a brick that you will have distinct seasons with large swings in temperature and humidity levels, some more so than others but they will all be noticeable.
What we can now describe for you is the behavior of wood to move with the seasons and as noted above, this is called cycling. Cycling is the measurable movement or expansion and contraction of wood from a humid climate to a dry climate and then back again into humid.
Wood in this environment can grow a surprising amount. Have you ever had a door that was easy to open and close in the dry weather become tight or difficult to open when the weather turns wet and moist? The temperature is not as big a variable as one would think; it’s the moisture content of the atmosphere that causes the movement. What is happening here is the wood is absorbing moisture and expanding to the point of closing up the gap around the door and stopping it from being operated in the usual way. Now, consider a joint between two or more pieces of wood that is glued with stiff glue like epoxy that has no flexibility. The wood will swell and the epoxy will stay static. It just doesn’t absorb moisture like wood.
This joint will probably look ok for some time, but over a few seasons the joint weakens like a piece of thin metal that has been folded and straightened many times; it becomes fatigued and weak. This is epoxy glues weakness.
The glue to use in these situations is glue that has a level of creep. This is the name given to the behavior of a glue joint becoming raised over time; it’s like the joint between the pieces of wood became squeezed so tight that the glue is forced out of the joint a little.
In some circles it is looked upon as a bad thing but if you understand wood movement you will see that at some level it is a desirable trait if the joint is to be good for a decent period of time. Glues like the Titebonds and the PVAs have the capacity to creep, and as I live and work in the wet tropics of far North Queensland, Australia, just below the Daintree national park, I look for a glue that can move with the seasons. I don’t know how long these glues can stay flexible, but they at least have the chance of outlasting the epoxy joint that is under climatic pressure from day one, at least in my climate.
The image above is a screen shot of page 7 in the PDF linked here.
It shows the moisture content of wood in different temp and relative humidity ranges. You should see your climate zones here. Click on image for larger view.
While I recognize the effects of cycling on wood glue joints, what is less certain to me is the fail point of the glue joints in question.
Is it the epoxy that fails or the epoxy/wood interface that tears. In my mind it is the eventual failure of the wood fibers that are closest to the epoxy.
Bear in mind that the joint may be fine for years before it just lets go.
I have had it happen to a collection of furniture that I had constructed out of one log of Tulip oak.
We moved from a high humidity environment that the furniture had acclimatized to, into a climate that was very dry for 11 months of the year, basically on the edge of a desert and the wood joints all suffered badly to the point that the furniture was unable to be salvaged.
It was a mess and the glue in question was epoxy. I suspect that a more flexible glue would have suffered also as the climatic change was so extreme. In circumstances like this, epoxy is not the best glue for the task.
So, the take away here is that epoxy is a good glue for wood, maybe even a great glue. It just depends where that bit of wood ends up living. Marine environments are the most stable. It seems counter-intuitive, however, once the wood has absorbed what atmospheric moisture it is going too, it becomes very stable and very predictable. The inland wood joint is just as predictable but far more unstable. The thing to remember is all glues have their place and are all good for wood joints. It is up to the diligent woodworker to consider the working environment of that wood and it’s associated joints and to then choose the appropriate glue.
Article written by Tim Blanch as part owner of Pickers Ridge Hardwood Designs. He is a qualified Carpenter/Joiner and has been turning wood for over 25 years and gluing things for longer.