What glue to use to repair this finger joint? - Follow-up Report

[Rushton]

In July I asked about what glue to use in repairing a separated finger joint in the pedestal base of an Ekornes chair. Phil S and Bill Simmons recommended using System Three T-88 Structural Epoxy. I followed their advice and thought I'd report back having now just finished the re-glue which is still curing but looks like it will work out just fine. I had a couple of "ah hahs" coming out of this effort that I thought might be worth sharing with some pictures.

I made up a simple caul clamping arrangement using pipe clamps and 2x4s that I'd cut coves into to match the radius of the base:

I thought the side-to-side clamping pressure would be all I need to pull the joint together, but apparently I should have added clamping along the other axis as well. You can see that the joint pulled together tightly along the inside edge, but is slightly open along the outside edge. My assumption is that the clamping pressure on just one axis slightly spread the base open causing that outside gap.

Tight inner circumference joint:

Slight open in joint along the outer circumference:

So, if doing this again, I'd know to added clamps along BOTH axis to keep pressure more evenly distributed around the circumference of the pedestal base. Thankfully, the T-88 epoxy seems to have good gap filling properties!

My other take-away from this project is to be prepared to thin the epoxy if needed and to have some lacquer thinner on hand for that purpose. The T-88 mixed up moderately thick and I had to work it into the splines with a brush to get coverage all the way into the bottoms of the spline joints. A very slight bit of thinning may have allowed the epoxy to flow more easily into those narrow spaces.

Thanks for everyone's contributions! Hope this may help someone else.

 

[Mike Davis]

That is why I always clamp my work and check the joints to see that nothing weird is happening. Backing off the pressure just a little or adding another clamp across the joints may have corrected that and you could have known before you applied the glue.

 

[Rushton]

Yep, thanks Mike. This is a learning process for me. This was a good one to learn from! From now on, I'll do the dress rehearsal dry fits of any joints I'll going to glue up.

I probably could have salvaged the fit of the joint during my glue up if I'd understood at the time what was happening (or what in retrospect I think was happening). In any event, this is why I posted my experience: we can all have a shared learning experience or a refresh on what we already knew.

 

[Wyatt Co.]

Thanks for sharing.

For future reference, after mixing your epoxy, heat it to thin in.

 

[Rushton]

Great recommendation! Thanks.

 

[Jeff]

A little additional information from the System 3 folks about their T88. System 3 is new to me so I started nosing around their information pages.

1.

Pot life of the mixedadhesive is approximately 45 minutes at 70°F. However, acoated joint may still be pulled up tight for two hours.

Maybe just relaxing the clamping pressure just a bit would've closed the joints while in progress but a dry fit with clamps is always helpful (per Mike Davis). Epoxy doesn't really need a lot of clamping pressure to begin with, maybe just a light snugging?

2. The mixed T88 has a viscosity of 9,000cps (honey is 10,000 and karo syrup is 5,000).

3. An 18 degree F rise in temperature cuts the cure time in half. Maybe that 9,000 cps viscosity was okay as is and using a brush to get in the tiny areas?

 

[Rushton]

...Maybe that 9,000 cps viscosity was okay as is and using a brush to get in the tiny areas?

A small stiff bristle acid brush did seem to work well to get into the T-88 into the tiny areas of the joint.

...Epoxy doesn't really need a lot of clamping pressure to begin with, maybe just a light snugging?

Good to know for another time! I'll be using the T-88 for two other projects and this experience with it will help.

 

[Wyatt Co.]

One can smear thick epoxy resin on joints, cracks, divits, and if it appears too thick (which is usually the case), to do the job you intend it to do, heat it with a hair dryer or heat gun.

ANY epoxy that is "heat set" is clearly marked on the package. Reason being is that for those who work with epoxies on a regular basis know to heat it to promote a more fluid material. The "heat set" is a warning to those of us who use epoxies on such a regular basis and know and understand this rule of thumb.

Heating most epoxies will make them run like water.