Gluing White Oak for Exterior Service

This discussion supplies good insight into the various ways glued joints can fail, and the reasons. February 1, 2015

Question (WOODWEB Member) :
Were working on an English tudor house job where we are having to glue up white oak for a finial. The final size will be 7 1/2" inches square. The final product will be left to weather and check, no finish. Anybody have any experience with epoxy vs. resorcinol? We had a delamination years ago in a similar situation with the epoxy delaminating.

Forum Responses
(Adhesive Forum)
From Contributor O:
So now I'm going to lay awake at night worrying about all those epoxy joints I have out in the world. I trained on Resorcinol, but changed over to epoxy in the 90's as a result of all the 'tech support'. Where can I get the colder setting Resorcinol?

From the original questioner:
I've been talking to boat builders, they are touting polyurethane as the way to go. Its flexible and no dark glue line, like Resorcinol.

From Contributor O:
I have gotten away from urethane - the big brand type with a large animal on it. It does not hold as well as other glues, and I just don't feel I can trust it. The foam doesn't bother me too much, but the strength is not there as I see it and through my tests and those of others. This makes me think of resorcinol as the long term solution. I have an exterior poplar fence post ball that was glued with resorcinol and the wood is mostly weathered/rotted away after 35 years, but the glue lines are still out there.

From contributor A:
Epoxy is just like other glues. You need to know how to use it. Epoxy does not like smooth surfaces. Planed and jointed surfaces should be roughed up with 80 grit sandpaper. Also thick glue lines are preferred over thin. PVA glues like smooth surfaces. The best bonds are between freshly jointed boards off sharp knives. Thin glue lines are superior to thick. PUR(Gorilla style) is not a great wood glue. Resorcinol is great glue; however, it is inconvenient.

From Gene Wengert, forum technical advisor:
The "resorcinol vs epoxy" report cited above says that the US Forest Products Lab has found that epoxy is weak, etc. However, that statement is reporting old formulations and is from a report about an improved formulation. The report actually states "The lack of structural durability of epoxy bonds to wood has always been a problem for fabricators of adhesive-bonded wood products intended for service in exterior environments. Although epoxy adhesives develop dry shear strengths that exceed the strength of wood itself, the epoxy bonds fail in delamination once exposed to the severe stresses of water soaking and drying. Recent research at the Forest Products Laboratory (FPL) demonstrated that a hydroxymethylated resorcinol (HMR) coupling agent physicochemically couples to both epoxy adhesive and lignocellulosics of wood to produce bonds of extraordinary structural durability." The report goes on to talk about boiling in water tests and so on.

It is well to remember the basic analogy of a glue joint is five links of a chain. The first and fifth links are the strength of the wood (sometimes a sanded wood surface has so many loose fibers that the joint will fail because the glue was not attached to solid wood but to the loosely attached fibers), second and fourth are the attachment of the glue to the wood (often a problem with teak and other woods when the wood's surface is not properly prepared, has oils, has aged, is not flat enough), and the third (middle) link is the strength of the glue itself (Remember that epoxy does not like a thin joint, but PVA does, etc.). When a joint delaminates, it can be a failure of any one of the five links. The previous cited report about resorcinol vs. epoxy gives many examples of epoxy joint failures and blames the adhesive. I have seen many glue joint failures with other adhesives, including resorcinol. Every adhesive used on wood has its failures. It is incorrect to assume that the failure of a joint indicates a failure of link three, the adhesive. Indeed, Contributor Adam has just posted some good suggestions about factors, in addition to glue failure, that can make a joint fail.

From Jeff Pitcher, forum technical advisor:
Excellent points Gene. Obviously Pardey's report is based on his experiences and lacks any real scientific basis. That said, sometimes personal experience wins out. I've always felt that both adhesives have a legitimate place in the marine industry. Pardey's experience, however, has indicated to him that the combination of salt water and warmth tends to negatively affect epoxy bonds. That said, I'm looking forward to reading the report you indicated.

From contributor M:
Canoe builders and mega yacht builders around the world have relied on West Epoxy for years and that was good enough for me for the last 26 years. Yes it's pricey but I have never had one failure when used properly on exterior work either on commercial work or boat work, fresh and salt water. Debates like these have raged on for years and everyone has their own opinion.

From Gene Wengert, forum technical advisor:
With both adhesives discussed here, if properly applied (correct joint thickness, proper mixing, curing temperatures, etc.) and with wood that is properly prepared (remember that the oils in teak and other woods must be cleaned off seconds before gluing and the surfaces must be free of lots of loose fibers, etc.), you can create a joint that is stronger than the wood itself. In fact, with both adhesives the joint is so strong that you can actually have a few small problems and lose a bit of strength and still be stronger. Having said this, I know that there are many people that apply adhesives incorrectly, such as applying too much pressure to epoxy or using old surfaces. It is these people that contact me (for over 35 years) about gluing failures. They always blame the adhesive and never their actions.

From Contributor B:
I've said this before but I have my own ultra-scientific test lab, an ever growing pile of glued-up scraps and cut-off pieces laying directly out in the sun and weather (occasionally I burn them in my fireplace). Here then is a historical fact: "They all fall apart with equal enthusiasm." It doesn't matter what kind of glue because it's not the glue, it's the wood. Any glued-up bare wood left out in the sun, wind and rain will fall apart (and sooner rather than later). Here's another fact: Any wood joint properly glued, sealed and (kept) protected from the elements will almost never fail If you really don't want to worry about glue joints, then don't have any. Order yourself a solid white oak tree trunk cutting and have your finial turned on a lathe. After that, you can positively guarantee (with confidence) that this solid turned piece will also (eventually) crack and fall apart.

From Gene Wengert, forum technical advisor:
It is not a good idea to burn glued wood in a fireplace as the glue does not burn cleanly but is likely to create carcinogenic chemicals. When you have a good glue joint, the weakest link will be the wood itself. The UV light and weather erosion will cause an effect in the outer 1/100" of the wood. Moisture and the resultant stress will cause more weakening. For a critical joint, plane the surfaces with sharp knives and good speeds to avoid heating and then glue within minutes. This will provide stronger joints by far which will last longer.