On the Mark: How to test and troubleshoot epoxy glue joints


Epoxy curing

A bit more seriously, the epoxy cures over the course of a few days to final strength. It is an exponential curve, so after the first 48 hrs you really can say it is at "full strength" under normal conditions (proper ratios and recommended temp range). Look to your epoxy vendor to provide the curves. I regularly trim up my joints at the 8-12 hour mark while it is a little bit soft.

Joint problems

The epoxy should create a joint stronger than the wood. I have not had any joint failures in samples that the wood did not break away. If the epoxy is breaking away form the wood, one or more of the following could be the problem:

Too smooth a surface. Your joints should be cleanly sawn or freshly sanded. If the wood was run over a planer or jointer, sand to 60-80 grit. Certainly DO NOT go finer than 100 grit. The planer will compress the fibers and burnish the surface, closing the wood pores, crushing wood fibers and generally impeding the bond. The slightly rough surface is required for a good mechanical bond. A freshly hand planed surface is also fine.

Improper mix - ratios and thorough mixing must be correct. Be sure to mix the resin and hardener thoroughly prior to adding fillers. I also always put the resin in first. There is more of it and if any is stuck to the sides un-mixed, it is less of an issue than if the hardener is stuck to the sides.

Did the leftovers in the mixing cup get brittle hard by 24 hours? I always leave my epoxy left overs sit in the mixing cup and check them to ensure that I did get a good mix and cure. My high tech mixing cups (partial to Cool Whip bowls) are cheap and if I find the glue is gummy there after a day, then there is a good chance that the entire batch was bad. You should be able to flex the bowls and pop out the uncured epoxy. The hardener does attack the plastic making it brittle after 2-4 uses.

Glue starvation - clamping too tight. This is not like using PVA (carpenters glues) it is VERY EASY to smoosh virtually all of the epoxy out of the joint. Moderate clamping pressure is all that you need. Having your gloved hands coated with wet epoxy is a good way to limit how much force you can apply with a screw clamp.

A problem to watch out for is that the wet epoxy is an excellent joint lubricant. A joint that is being clamped will generally slide sideways as pressure is applied. It won't stay put until almost all of the epoxy is squeezed out (warning sign) out or it starts to cure. I often add a few screws, nails or staples to stabilize the joints laterally while clamping. Otherwise, I quickly run out of hands, clamps and have a big mess.

Glue starvation - epoxy wicked out of the joint. This is normally a problem when gluing end grain or steeply angled grain. With porous woods, it is a good idea to brush on some unthickened epoxy on the joint surfaces (or at least, all end grain surfaces). Then proceed to add your thickeners/fillers.

Unthickened glue. The raw epoxy is rather brittle, the fillers not only extend the epoxy (increase its volume) and make it thixotropic (stays put without running). They also make it much more resilient. Think of the pure epoxy as concrete without aggregate (stones, rocks, gravel, sand, fibers) added. It will be easy to fracture. Adding the fillers greatly increases the strength. Just compare a drop of hardened resin with one that has some high density silica in it. The one with silica is MUCH stronger.

Glue ran out of the joint. Used unthickened epoxy, and too much clamping pressure on a vertical joint.

Surface contamination. Wax, oil, silicone, grease, old finish, will all prevent a good bond.

End grain to end grain joint. There is no way to make a strong glue joint here with ANY gluing technology. Mechanical help is needed to get to having a side-grain to side grain joint. Use a mortise and tenon, half laps, simple lap, floating tenon, spline, gussets, etc. Biscuits are not recommended as they are made form beech which rots very easily.

What to do?

The areas I take the greatest care in are:

  1. Proper joint preparation: clean joint, no contamination, no planer marks, no burn marks
  2. Pre-drill for screws or have the nail gun ready to secure the wood for lateral stability
  3. Mix accurately - I ALWAYS use pumps and mix thoroughly before adding fillers
  4. Clamp only as tight as I need to hold the joint together
  5. Check the bowl the next day for a proper cure

We are very pleased to have Mark sharing his expertise with his fellow boat builders. If you would like to comment on this article to Mark, he can be reached at "builders at bronkalla.com". Replace the at with @. (Just trying to dodge a few spammers). See Mark's great web site for information about his Riviera project.

More information about epoxy