Epoxy Versus Urea-Formaldehyde Resin for Glueing Oily Wood

      Lumber and surface preparation may be more important than glue choice for good performance. But this discussion does shed some light on glue characteristics as well. February 16, 2014

I finished gluing up a 6/4 3'x5' Jatoba slab that will be in an exterior setting. I had 2 joints split half way after 48 hours and was able to break the others loose with a rubber mallet. I used West epoxy and glued up the joints within 1 hour of machining. The broken joints are all along the glue line and there are no torn wood fibers.

After searching the archives I think I should have:
1. Sanded the edges with 80 grit paper,
2. Wiped them with acetone prior to gluing,
3. Backed off the clamp pressure a bit to avoid starvation.

Should I try the second round with epoxy or should I switch to UF resin? Is one better than the other for oily woods? I did read about the possibility of epoxy being damaged by sunlight, but I'm not too concerned as this will be shaded and in a relatively protected area.

Forum Responses
(Adhesive Forum)
From contributor T:
I have used epoxy on cedar, but it is a pain to sand if you get squeeze out. Clean up with acetone is not fun. I have used Titebond III on teak on a stress point and it has never opened up in a four-seasons setting. It is easy to sand off, but it will repel/show through stain. I have used polyurethane glue (Gorilla) on resawn cedar planks to create curved boards and that has never opened up in 8 years of four-seasons weather. I have never used a UF glue.

From contributor A:
We have used Jatoba on several projects over the years, including the desk I'm sitting at now. No open or failed joints with CP adhesives mp2. Have I just been lucky?

From the original questioner:
I didn't see an mp2 glue on the website. Are you referring to the CP-0203 PVA glue or am I missing something?

From Jeff Pitcher, forum technical advisor:
The CP product in question is CP-0201 and CP-0200. Both are crosslinking PVAs and both will work just fine on jatoba. That said, what this thread really shows is that it's not so much about the adhesive as making sure you have prepped the wood properly. This includes making sure the moisture content is 7-10%, the wood is freshly machined, the spread is correct, the pressure is correct and the gluing conditions are all appropriate for the adhesive being used. If all else is done correctly, you'll be successful using PVA, polyurethane, UF, epoxy or others.

From the original questioner:
Thanks. I made some samples today using epoxy and no clamps after sanding and an acetone wash on the joint. I plan on doing some other samples in the morning with PUR and PVA. The moisture content averages right at 7%.

As far as prep work, if I were to use a PUR or crosslink PVA, would you still recommend sanding the freshly jointed edge and cleaning with acetone or should a freshly jointed edge (within 1 hour) work just as well? I'll be changing out jointer knives to make sure I have a fresh edge.

What's the difference between an adhesive and a glue? Is there a difference in the proper joint preparations for each? Is epoxy the only one that works better with a thicker joint?

From Jeff Pitcher, forum technical advisor:
A freshly machined joint should be just fine. These days "glue" and "adhesive" are used pretty much interchangeably. Historically, glue referred to natural products and adhesive referred to synthetic. As far as I know, epoxy is the only adhesive that works as well with a thick glue line as a thin one. As for the surface prep for polyurethane vs. PVA, I would say that a PUR will generally be more forgiving of less than adequate surface prep.

From contributor J:
The only time I ever consider using epoxy is when I can't get a single clamp on something. A well clamped and thin glue line using epoxy is a recipe for failure. Otherwise I think it's fair to say that any quality commercial glue (be it yellow, white, brown or purple) will positively stick your stuff together. In fact, the only glue that I am pretty sure will fail after being pressure clamped is epoxy. Another fact... The only glue that I am pretty sure will never fail with no pressure clamping is epoxy.

From contributor A:
Sorry, it was Custom Bond MPA2. All joints were either glue joint rip or cope and stick cabinet doors. No sanding or wiping with acetone.

From contributor S:
UF glue does not stick to everything. It will allow for about a 1/16" gap fill. Polyurethane will stick oily wood together, but really doesn't have much strength. I have used Titebond on jatoba with good luck. It doesn't seem all that oily of a wood to me. I don't ever remember wiping it with solvent prior to glue-up, but that might be my halfzeimers doing my remembering for me.

From the original questioner:
Thanks for the replies. I ended up using PUR after fresh machining and moistening both sides, and it worked just fine. I made a few extra 6" long samples at the same time to make sure. I hit them with a hammer and none of them broke along the glue line, so that's good enough for me.

I also did a few samples using PVA and it worked well, and for some reason I did get some glue line failures, but there was a 1/4" strip of glue line failure then 1/4" strip of wood failure, and it repeated like that. I'm guessing it has something to do with the growth rings?

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