Troubleshooting Epoxy Joint Failure

Epoxy has its own little tricks. Here, woodworkers provide advice concerning a failed epoxy glue-up. August 23, 2006

I recently glued up a bowsprit for my boat using West System. I believe it was the 207 slow hardener. It is a beautiful piece of work and sat in the shop while I oiled it (soaked the oil into it with rags). I was on the way to the boat to install it and left it out in the rain overnight (by the way, the harbor I am in is in Whittier, Alaska and has sometimes continuous rain for months on end, which eventually turns into snow). There were about 4 glue joints that just gave up without any stress on them and literally fell apart. Our joints were planed, not sanded. We applied a fair amount of pressure. When looking at an actual failure, one side had epoxy residue on it and adjoining piece had none.

Forum Responses
(Adhesive Forum)
From Jeff Pitcher, forum technical advisor:
It sounds like you're not getting enough pressure to transfer the adhesive to the opposite side. Or your joints could be burnished or untrue. However, it sounds like you took great care in the preparation. Most folks underestimate the amount of pressure necessary for a good glueup. Also, you might run it by the guys at West System.

From contributor K:
What kind of wood were you using? Epoxy, unlike most glues, does not like a lot of clamp pressure. It likes a fat glue line. When you are using it as an adhesive, you are better off wetting the wood with un-thickened, allowing it to penetrate, then following that with thickened epoxy and light pressure so as not to squeeze it all out of the joint. If your jointer or planer was dull, or the surface was not freshly planed, you may have had a burnished or compressed, or otherwise self-sealed, surface that did not allow the epoxy to penetrate the surface. You may want to consider a less permeable finish than oil to slow down moisture exchange, also.

From contributor E:
One more consideration is mixing and ratio. West epoxy likes its ratio to be very, very accurate. Epoxies in general also need to be extensively mixed. The hardener and resin do not combine easily. I've seen many people stir a batch for 30 seconds, use it and wonder why it failed. It really needs several minutes or more depending on batch size.

From contributor L:
Two more process parameters are of interest. One is the open time of the adhesive. From the observation that only one substrate showed all the adhesive upon failure, it can be deducted that this is the substrate the adhesive was applied to. Before the joint was closed, open time might have been exceeded. Some epoxy adhesives also show some sort of oxidizing or reaction to air, giving it a surface which is less likely to wet out the substrate. Applying the adhesive to both substrates minimizes this sort of problem. Two might be the moisture content in the wood as well as the RH (always rainy?). Epoxies do not behave well in this aspect. Above 70% RH epoxies should not be used on wood.