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Aug 12, 2019, 11:54 AM
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Are your glue joints repairable


In 2005 experienced woodworker Bob Smalser published the results of his tests on the subject of what glues effectively stick to cured glue joints that have broken in a series of discussion threads in the online forums of sawmillcreek.org, finewoodworking magazine (both woodworking forums) and woodenboat, a forum/magazine on wood boats.

If you google "are your glue joints repairable", these threads should surface. Here is a link to the sawmillcreek thread, the longest of the threads:
https://sawmillcreek.org/showthread....nts-Repairable

Skip to either post #43 on page 3 or post #86 on page 6 for a summary of his findings.

He tested a number of cured joints needing repair, including epoxy, polyurethanes and aliphatics (yellow/Titebond glues). He did not test a cyanoacrylate joint needing repair but did test CA as a possible repair glue.

As I read his findings, most of the above broken glue joints are repairable with epoxy and that a broken aliphatic glue joint is repairable with CA but not with epoxy or with more aliphatic glue.

My takeaway is that unless you sand/abrade a broken joint to get to "uncontaminated"/fresh wood, take care what glue you use to repair wood joint damage after a crash. A repaired wood joint with the wrong glue - or more of the same glue (apart from epoxy joints apparently), while it will probably "stick", will not be as strong as fresh wood to fresh wood joint - where, with almost all glues, the joint breaks away wood fibers, not just at the glue line.

Michael in Ontario, Canada
Last edited by 2michaely; Aug 12, 2019 at 01:58 PM.
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Aug 13, 2019, 03:33 PM
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Absolutely. You just can't glue over another glued surface. I have applied epoxy over a polyester resin surface with no problem but the surface needs to be scuffed a bit.
Aug 13, 2019, 04:16 PM
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GeoffS's Avatar
The WEST System epoxy users guide recommends 80 grit sandpaper on most non-porous surfaces to get good "keying" (i.e. a mechanical connection between the new glue and the existing surface).
https://www.westsystem.com/instructi...e-preparation/
I've always followed this guideline when I'm gluing to a surface that I know will not make a chemical bond with the new glue.
Aug 14, 2019, 03:21 PM
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Quote:
Originally Posted by GeoffS
The WEST System epoxy users guide recommends 80 grit sandpaper on most non-porous surfaces to get good "keying" (i.e. a mechanical connection between the new glue and the existing surface).
https://www.westsystem.com/instructi...e-preparation/
I've always followed this guideline when I'm gluing to a surface that I know will not make a chemical bond with the new glue.
Indeed. I use West Systems for a number of gluing tasks when I don't trust the typical "hobby epoxy" to do the job.
Aug 25, 2019, 09:03 AM
Rust: The poor man's Loctite.
TomM's Avatar
I've been a member of Sawmillcreek for 15 years. Some of those guys need a hobby lol.

In all my decades of modeling, I can't remember a glue joint failing, with the exception of a bad batch of Bullet brand CA from Tower Hobbies back in the 1980's where almost every joint on the airframe just fell apart.

In furniture making, I've had glue joints fail due to seasonal contraction and expansion.

But in the model airplane repair shop, it's almost always the wood/plastic/composite right next to the glue joint that cracked. If something can't be "puzzle re-joined" and glued next to the failure, I just cut out the assembly and rebuild as needed. From a young age I learned first hand that "glue on glue" is just poor workmanship.
Aug 25, 2019, 02:17 PM
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As above.... Never actually had a glue joint fail.
Always the stuff around it.
A strange concern.. imo
Aug 26, 2019, 04:58 PM
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Quote:
Originally Posted by Bare
As above.... Never actually had a glue joint fail.
Always the stuff around it.
A strange concern.. imo

TomM and Bare,

Yes, your point is well taken: typically, when joints fall, "fresh" wood is exposed on both parts of the break, so one need not be concerned about dried glue contamination and more of the same glue not working.

But in some joints, end grain to side/surface grain, which are weaker to start with, some of the time/often (?), the break does occur at the glue joint, at least on the end grain part. For example, rib ends to leading and trailing edges, some parts of fuselage formers to fuselage sheeting, etc. So in such cases you do have one surface "contaminated" by cured glue and in those cases, the caution of the linked article - that more of the same glue in some cases, including the yellow carpentry glue I mostly use, will not provide for a good repaired joint.

Michael in Ontario, Canada
Aug 28, 2019, 01:38 PM
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Most all wood glues work by wicking into the wood xylem and phlegm tubes and drying, providing a mechanical bond. If the joint fails in such a way that those tubes are still filled with glue, regluing will produce a weak joint, because there is inadequate absorption surface. Sanding off glue exposes at least some tubes so the glue can work. Same for gorilla glue (foaming versions), they expand into open cells of the foam. That requires sending surface to open the cells. GG on skinned foam or smooth continuous surface is very weak joint.
Aug 28, 2019, 06:33 PM
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Quote:
Originally Posted by 2michaely
TomM and Bare,

Yes, your point is well taken: typically, when joints fall, "fresh" wood is exposed on both parts of the break, so one need not be concerned about dried glue contamination and more of the same glue not working.

But in some joints, end grain to side/surface grain, which are weaker to start with, some of the time/often (?), the break does occur at the glue joint, at least on the end grain part. For example, rib ends to leading and trailing edges, some parts of fuselage formers to fuselage sheeting, etc. . .
Michael in Ontario, Canada
I was thinking the same thing as I read the discussion. In restoring the fuselage of my 1/4 scale L-4 there were many dozens of broken CA joints. Some were end to cross grain as you described and some were spruce overlapping balsa. I sanded them back to bare wood to repair. If I still had a glaze of CA then the repair was done with 1 minute Z-Poxy. I'm a big fan of WEST but individually most of the joints were not highly stressed as long as all of the joints were sound and I just didn't think in these repairs there was much difference between WEST and Z-Poxy. Once saturated with CA not much will soak into the fibers unless all of the hardened CA is sanded away.
Aug 29, 2019, 07:49 AM
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Quote:
Originally Posted by TomM
I've been a member of Sawmillcreek for 15 years. Some of those guys need a hobby lol.

In all my decades of modeling, I can't remember a glue joint failing, with the exception of a bad batch of Bullet brand CA from Tower Hobbies back in the 1980's where almost every joint on the airframe just fell apart.

In furniture making, I've had glue joints fail due to seasonal contraction and expansion.

But in the model airplane repair shop, it's almost always the wood/plastic/composite right next to the glue joint that cracked. If something can't be "puzzle re-joined" and glued next to the failure, I just cut out the assembly and rebuild as needed. From a young age I learned first hand that "glue on glue" is just poor workmanship.
I repaired a number of dining table chairs that had been nailed, screwed and glued back together in what amounted to a major rework. Whoever owned these chairs obviously had no clue. I removed a more than a dozen screws and nails along with improperly applied cheap glue, sanded every joint back down to bare wood and then re-glued and clamped.
I've never seen such a mess. You probably have though.
I repaired a leg on an old Hoosier cabinet I owned that had broken with West Systems epoxy . After applying the epoxy and clamping for 48 hours, the leg was like new and as far as I know the glue joint has never failed. One of my nieces received it as a wedding gift.
Aug 30, 2019, 10:52 AM
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GeoffS's Avatar
Quote:
Originally Posted by David2011
... I'm a big fan of WEST but individually most of the joints were not highly stressed as long as all of the joints were sound and I just didn't think in these repairs there was much difference between WEST and Z-Poxy. ...
WEST is great, but certainly not needed in every situation (I usually use cheap 5-min epoxy for foam models 'cause I'm just impatient..).

One advantage of WEST (and other "pure" epoxies) is that they don't contain any fillers. This allows you to choose the filler(s) and filler:glue ratio that best fit the job.
For R/C stuff, where weight is important and stress is generally low, I really like WEST's 410 Microlight. It's designed for fairing, but is still pretty strong compared to the loads on a typical R/C part. The next step up in strength is 407 Low Density filler which is even described as "...reasonably strong on a strength-to-weight basis."

Aside: WEST also sells an unfilled 5-Minute epoxy that you can add fillers to.

Quote:
Originally Posted by David2011
... Once saturated with CA not much will soak into the fibers unless all of the hardened CA is sanded away.
Basically a glue surface is the same as any other nonporous surface.
If the glue is well bonded to the substrate, then it's just fine to rough the surface with 80 grit sandpaper to give the new glue something to "key" into.
If the glue isn't well bonded then it really does have to be removed (the same principle as surface prep. for painting).
Aug 31, 2019, 10:41 AM
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Quote:
Originally Posted by GeoffS
...



Basically a glue surface is the same as any other nonporous surface.
If the glue is well bonded to the substrate, then it's just fine to rough the surface with 80 grit sandpaper to give the new glue something to "key" into.
If the glue isn't well bonded then it really does have to be removed (the same principle as surface prep. for painting).
Curious.... have you read the Bob Smalser threads that I cited in first post?

Michael in Ontario, Canada


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