Let's talk epoxies....

[KLaz]

So I have a couple complicated glue ups coming up soon and instead of relying on ol' yellow glues, I'm wanting to use slower setting epoxies.

I understand West Epoxy with the resin and hardener.  Any of you out there using any of the fillers or additives they offer such as the micro fibers?

I've also seen the System Three product out there too.

Just curious to hear some feedback on your experiences....

[Cooler]

There are different speed hardeners.  Some vendors allow you to specify the hardener speed.

Read:  https://boatbuildercentral.com/EpoxyResinCureTimes.pdf

Epoxy mixed with hardener will cure in a wide range of temperatures,but the speed of the cure varies with the hardener. We sell three hardeners: slow, medium and fast. They can be mixed for even more precise cure speed.

[Alan S]

I haven't use West, but have used at least six different System Three epoxies. I imagine the West range is similar. Fillers and additives serve various purposes, but the microfibers are designed to thicken a runny epoxy so it won't drip out of the joint, and can strengthen it. When gluing wood with epoxy, I prefer what I think of as the old fashioned approach, since it uses the basic resin, rather than one specifically made for gluing.

Mix up the fairly thin epoxy (at a precise ratio and very thoroughly), paint it on the surfaces to be joined, and let it soak in for a few minutes. If it looks dry (as on endgrain) paint on more. Now mix the microfibers into the rest of the mixed epoxy, paint it on the joint, assemble and clamp.

Gluing an 8:1 scarf joint this way leads to very strong canoe gunwales easily made 20' long, that can be subsequently bent to shape without heat. Keep in mind that epoxy, while dry after a certain amount of time (dependent on the type) continues to get stronger for quite a while. I usually give it a week. I suspect the gunwales could have broken at the join if stressed immediately on hardening, but are close to the strength of the long grain wood on full cure. (That's a heck of a lot stronger than what is usually meant by a glue bond being stronger than the wood, since that describes side grain strength -- what cleaves on splitting.)

Strength depends on using the proper ratio of resin to hardener. When you assemble furniture, you want an equal number of tenons as you have mortises to build the strongest structure. If you think of the resin as having mortises, and the hardener tenons, it makes sense that you won't be able to build as strong a cured epoxy structure if the ratio is off. Make sure you mix at the ratio described by the maker of the epoxy you use.

People also worry about overclamping epoxy joints, squeezing out the epoxy, and being left with a weaker joint. I think the critical feature that can lead to such failures is not the force of the clamp, but the way it's applied. Ordinary PVA glue grabs, so if you clamp and then rearrange the clamps, the joint is likely to stay together during the process. Epoxy does not grab, so any movement after squeezing the joint together is apt to cause loss of intimate contact in the parts after you've squeezed out excess epoxy, leading to a weak or no join. As epoxy is better at gap filling (and with microfibers, much better) you don't lose strength by clamping less. But with care you can clamp to invisible joins that are strong.

[KLaz]

Great information Alan.  Thanks!

[Willyou]

I recently posted about my experience in repairing an old rocker using epoxy. All the joints were very loose and sloppy. So, thinking that epoxy is good at filling gaps, I used West Systems T88 for all the repairs. It failed miserably. A West Systems Tech person told me that regular epoxy is a glue intended for typical snug fitting joints and not intended for what I tried to do. For that you need a filler or use their Gel Magic product. I took the chair apart and cleaned up all the joints and, this time, I filled the epoxy with sanding dust to make it a little thicker than peanut butter. This worked much better.
Before my final glue-up, I did some tests gluing some sloppy joints with both the thickened and unthickened epoxy. The unthickened joint came apart with minimal stress. The 3/4" dowel just levered out of the enlarged hole. With the thickened epoxy, the dowel broke just above the hole.

[mike4244]

So I have a couple complicated glue ups coming up soon and instead of relying on ol' yellow glues, I'm wanting to use slower setting epoxies.

I understand West Epoxy with the resin and hardener.  Any of you out there using any of the fillers or additives they offer such as the micro fibers?

I've also seen the System Three product out there too.

Just curious to hear some feedback on your experiences....

Dap urea formalahyde glue has a long open time

[Mike L B]

FYI
T88 and Gel Magic are System Three products, not West System.

[Willyou]

FYI

T88 and Gel Magic are System Three products, not West System.

Misspoke. Thanks for the correction.

[toolmiser]

Since we are on the subject, is there a way to make it thinner, or flow better? There are times I want it to go down into a crack that I am trying to repair. It tends to flow so slow that it hardens before it can get into the crack.

[Willyou]

Most epoxies will get quite runny when warm. Warm up the two components before mixing in a bath of hot tap water. You can also warm up the wood piece you are working on with a hot air gun. Start with a slow setting epoxy as the heat will accelerate curing.
Some say that adding a solvent like lacquer thinner will also do it. I'm skeptical. I'm not sure but, I think, it will weaken it.
If you have a crack that goes all the way through, you can put your vacuum on the other side as you apply the epoxy and the suction will draw it through the crack.