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Epoxy has more dampening effects that most other glues. I use Tite Bind and HHG. I see no reason to change and I don't see any advantage to epoxy. You would be better of using CA . Also the epoxy will fill gaps but again , that isn't a good thing on bridges as your joinery on this parts needs to be pristine.
I enjoyed this thread . If you learn one thing it is that given 3 luthier the same question you may get 4 opinions

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

John, I suspect that PVA has more dampening effect that epoxy. What do you base that on? I'm basing my assumption on my own very good opinion.

Assuming a good joint for these materials, do you think there would be any practical or noticeable difference on the guitars tone/performance/longevity/sustain or anything else that matters?

(Questions not just for John!)

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Try finding a 15 blade and cut with the sharpened edge rather than the point. Would be hard to do in some cases though. A 10 blade would work too but it's physically larger.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

I suspect a good structural epoxy is quite low damping. This is based on experience building aircraft spars using carbon or s-glass.

I can also add that a carbon/epoxy windsurf mast held up in the wind at a nodal point will vibrate quite nicely producing a loud hum. I also check 2-piece masts for fit by tapping on them after assembly. when they are put together correctly they ring like a gong with a nice sustain.

-jd

Regarding epoxy

well thats my take and me sticken to it a lot better than epoxy wood.

Like why not just pop rivet the sucker down and be done with it.

so there


blessings

Padma

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Audiences and dispensations on Thursdays ~ by appointment only.


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Epoxy dries softer than Tite bond and white glue, The bond joint is usually thicker also with epoxy. I have been building for over 11 years using HHG tite bond and White glue. I had 1 failure and that was due to not getting down to clean wood and was early in my career. I have been routing the tops to glue on the bridge and have not had any failures since .
All the bridge designs I use are using ball end strings. I don't use a pinless bridge.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Part 1 is incorrect, part 2 is correct. They're related to different properties of epoxy, though. It's an adhesive and it works just like any other adhesive in that regard. What people don't understand is that wetting is really just conforming to the surfaces at the cellular/molecular scale. It's the real 'mechanical bond'. Toothing seems like a great idea until you actually understand what's going on.

Epoxy as a material is strong and quite cohesive, and that is a useful property. In the end, though, the part acting as an adhesive and the part acting as a plastic might as well be seen as different things entirely. For all intents and purposes, using epoxy to fill gaps is equivalent to gluing both surfaces to a plastic shim: it works, but only if they're glued to it properly!




An adhesive manufacturer (formulator, actually) giving bad advice, shocking! Solvent wipes on porous surfaces are pure hogwash. Planing leaves a less active surface than sanding; even though it looks good macroscopically it's burnished and not properly sheared on the microscopic scale. If your test results came out the way you expected, then there was an error in methodology.

I didn't pull this stuff out of the sky, there's a mountain of proper research and testing to be read by anyone looking to learn.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Fire away with the links, Bob. The info about 'surface activation' and 'wetting' didn't address the question here: "Does epoxy work as an adhesive primarily by penetrating into the substrates and then curing into a strong 'linking material', or does it chemically bond to the substrates?"

John

This bridge gluing thing is a pet peeve of mine.
From what I have seen, an instrument's practical lifespan is often determined by it's bridge joint history. A top starts out life very close to it's minimum required thickness. All efforts should be made to preserve it. There are only so many times a bridge can be reglued with modern adhesives before too much top has been removed to get to bare wood. Once the top is too thin in the bridge area the only solution is a re-top and for most Instruments that means game over.
I have re-glued what seems like a lot of bridges in the last five years for all sorts of reasons. There are batches of Martins,(and others) floating around out there with bridges glued well but too far North. There are bridges that are cracked between pin holes, ones that have been shaved down too far, cupped from drying out, peeled because an employee or maker sliced fibers of top at the back edge, saddles so tall they broke out the front, Cedar with run out that gave up the fight, glue that skinned over before it was brought together, or insufficient pressure was applied to make the glue line thin enough, finish that was not completely removed from the top, Ebony stain left on the bridge bottom, and of corse the common automobile solar oven treatment. It goes on and on. To think that your guitars will never need the bridge re-glued at some point is not realistic . Even if you did everything right.
The rebated bridge thing is a strange idea that has gotten out of control. I have seen people come in thinking their bridge was lifting and tried to explain the part that is lifting never was glued down. It's just that despite their sound hole humidifier, the bridge is still dry and cupping. I have seen otherwise high quality guitars with finish left on 1/4" inboard of the back edge!
What is the point of worrying about the sonic quality differences between glues or paring away extra grams of bridge weight when the most stressed part of it,(the outer edge) is glued with nothing? It's like putting a heavy, decorative trim ring around a bridge just to hide the joint. If that portion of the bridge is so unimportant why not just take it off?
I have done it on pristine instruments. It takes time but is not that hard. put on your opti-visor everyone, score carefully, scrape back carefully, now glue that whole sucker onto the top with hot hide glue!
All guitars with lifting bridges look like crap.


Now I'll slap that submit button and take a breather.

A bit off topic, speaking of gaps.
A friend brought in a campfire guitar 3 years ago when I just started building, a classical fitted with steel strings. Bridge was knocked and lifting for years now,it ripped some of the spruce, and someone stuck some nasty goo in there. It was a mess. I put off the bridge, cleaned as much as I could, made a new rosewood bridge and stuck it on with LMI white. Wood to wood, maybe 40% of the surface, the rest gaps from the ripped spruce. At that time it didn't occur to me that PVA has poor gap filling properties but honestly the bridge holds perfectly 3 years later with medium gauge and subjected to some pretty nasty mountain conditions. I later glued cracked sides twice and a top separating from rim on that poor beater, but the bridge is solid

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Build log

Maybe. Epoxy ranges in hardness from Shore D 50 to 95.
Hide, LMI white and Titebond are around 80.
BTW - CA is often measure in shore A scale, which is for softer materials than the D scale. It is interesting to note that the leading manufacturer of CA makes a "toughened" product that adds a bit of rubber to it. This makes CA less brittle. I imagine that would dampen sound a bit!


This is true. An epoxy bond will be thicker than most glues. The implied assumption being that this is a bad thing. Is it? In my experience, to my ears, it is not. I could not notice an adverse effect in tone and sustain with epoxy.
Like anything else in guitar building, YMMV.
Just for the record - In my mind the best glue for the bridge is hide glue. That being said, I don't think it is the only glue that anyone should ever use.
Bottom line - In a well executed joint, any good adhesive will last a very long time in a guitar that is well cared for.


For repairs in general - one thing that has not been mentioned is the great advantage of hide glue and epoxy is the ability to re-glue. These adhesives stick to themselves very well.
This is not true of LMI white and PVA glues. (I'm not so sure about CA but I'd want to remove all of it for a re-glued joint.) The glues must be remove for a good repaired joint. It is always best to use the same glue for a repair.
Epoxy is the exception to this rule. Epoxy sticks well to itself and to other adhesives. This makes epoxy very useful for repairs when you can not remove all the old glue and/or you don't know what the old glue was.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Wow, this thread is really getting good!

Wish I had time to respond more to several of the things that have been said, but I've gotta go make breakfast for my girls.

Thanks, everyone, for all your input!

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

This thread sounds like a perfect example of the tail wagging the dog. As I understand it, because you haven't been able to remove the finish to a level never before achieved, you therefore want to develop an entirely new system to adhere the bridge to the top's finish. For 95% of the builders looking at those bridges, I'll bet that the "defect" that you showed was acceptable. While I realize that you are still in the information gathering phase, the purpose of the experiment is still questionable in my mind.

Your choice of epoxy was decided because it was the only glue which will adhere to the finish you are using. Along the way, you discussed and dismissed the traditionally used glues in order to justify in your own mind your choice for epoxy. Part of that rationale was your statement that HHG and others don't re-adhere to itself and that Epoxy does. In the case of HHG this is erroneous and misleading and really needs to be corrected. HHG DOES adhere to itself...fresh HHG wets and reactivates itself and amalgamates with the new glue...that's what makes repair of old vintage instruments possible. To be clear, I'm not suggesting HHG over finish...just correcting the record.

HHG has been used in gluing bridges and braces for many reasons not even stated here yet, but the most important property being its resistance to creep. One suggestion, do some research into those properties to compare the two adhesives...better yet, some testing to simulate shear and tension forces should be a minimum of prep work before going live on a customer's guitar. This should be easy enough to do and would be of interest to many of us.

HHG has been used for centuries on stringed instruments for many of the reasons others stated. The repair issues should be enough to give pause IMO. I would hope that new builders reading this thread look at the gold standard of adhering bridges with HHG before jumping into a radical approach such as you are proposing...and for a reason that is questionable after having seen your fine looking bridges.

Finally...we should all get our ears to perk up when we see Alan write the words, "Ovation"..."Epoxy"...and "Crap" in the same paragraph. Sometimes even the very best of experts in our field don't have the ability to properly articulate their opinions despite their extensive scientific vocabulary!

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JJ
Napa, CA
http://www.DonohueGuitars.com

just a couple of comments:

in the pristine world of luthery, a well-wetted minimum thickness joint with smooth surfaces and no voids will be strongest regardless of your choice of adhesive.

West Systems is primaraly a boatyard epoxy. I you are glueing metal to gelcoat or FRP or similar materials, you are going to have really lousy wetting angles and smoothing the surfaces can make these wetting angles worse. In these cases, you are going to get cohesive bonds -the bond will not be stronger than the material being bonded. In this case of cohesive bonding, roughning the surfaces to provide a tooth can increase the bond strength. Roughning the surface can also increase the resistance of the bond to peeling. Peeling is another problem with cohesive bonding. You do not want cohesive bonds in guitars.

The link i provided earlier to sawmill creek was premarily poking at the poor repairability of PVA glues such as titebond. As stated, PVA glue does not stick well to PVA glue.

-jd

Thanks for your input, JJ. Looks like there have been several points misunderstood, however - probably my fault in not expressing myself clearly.



It would only be adhered to the finish around the edges, outside the area that had been masked for finishing.

A more flawless look is certainly one of my motivations, and I make no apology for that (I also re-iterate that I would never put flawless look ahead of structural integrity, sound, or any other quality that is clearly a higher priority in instrument making), but I'm actually more motivated by time savings (ditto about never putting that ahead of quality).



No, but the fact that it would adhere to the lip of finish around the edge as well as to the wood in the area that had been masked is a significant part of the equation. It's gap-filling ability and cohesive strength are the main reasons for considering it in the method I described.

CA might work equally well in these regards - I'm not sure.



I haven't dismissed anything, nor do I feel the need to justify anything. I am examining an alternative method.



Actually, I didn't say that, but I might have been unclear in what I said. I was referring to epoxy's cohesive strength when used in gap-filling applications, which, as I understand it, neither HHG nor PVA have.



Repair issues do, indeed, give me pause, and I will consider that carefully in deciding whether to use this alternative method.

If you think I have a dismissive attitude toward traditional methods or conventional wisdom, you've got me wrong. On the other hand, I think there are times when questioning traditional methods, examining alternatives, and, in many cases, employing new or alternative methods and materials is well worthwhile. Indeed, it's traditional to do so. Steel string guitars wouldn't exist otherwise.



I'll take that as a compliment!
Seriously, though, no approach should be jumped into, whether traditional or innovative. I'll bet there's quite a few bridges that have come off because someone jumped into using HHG without mastering the technical aspects of using it!



Thank you, JJ. Much appreciated.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

I really want to respond to some of the other posts - lots of good stuff to consider and discuss - but I just can't take the time right now. Later...

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Bob Garrish wrote:
"Planing leaves a less active surface than sanding; even though it looks good macroscopically it's burnished and not properly sheared on the microscopic scale. If your test results came out the way you expected, then there was an error in methodology."

I'm not sure if that was a typo, or a mistake.

A _properly_ planed surface, be it by hand or machine, is sheared off cleanly, and not at all burnished. Dull sandpaper (or a dull power planer) will leave a burnished surface, though.

Carleen Hutchins used to cite a Forest Products Lab study from WWII that looked into the gluing up of wood propellor blanks. They found that the ones that were sanded to thickness tended to come unglued while the planed ones did not.

I was pretty sure that one wouldn't fly with you, but it was neither. The short of it is that there's no such thing as a cleanly sheared surface. At the micro scale, abrasive removal is the same process as planing, but with sharper cutters and less compression of adjacent material. There are a lot more things to mess up using an abrasive process which can quickly make it inferior to a cutting process and I agree that a planed joint made by a sharp and well set up plane is superior to what most abrasive setups use on wood will product.

So far as the studies go, neither of them would have failed if they'd made clean abraded joints and cleaned them up properly. Abrasive processes leave a lot of microscopic junk all over the material and in any pores, and it'll hold itself there with static if nothing else. The material needs to be thoroughly cleaned with high-pressure air or cleaners afterward ,and during cutting if it's something that'll clog the abrasive or get re-processed. If that isn't done, then that 'dust contamination' will totally ruin the wetting of the surface.



Todd got to that one right away. Epoxy holds nearly the whole market for gluing together non-porous substrates. The reason that all the research talks about wetting and surface activation is because 'mechanical bonding' was debunked a long time ago in that community.

I'm a big experiments guy, and there's a simple experiment to prove mechanical bonding isn't important: try to glue two pieces of UHMW together. Tooth them, sand them, slice and dice them, but the epoxy still won't hold them together as it can't form a chemical bond with UHMW.

As a related aside, I did an experiment for solvent wipes on wood by wetting one half of a cherry board with black-dyed oil and then trying to wipe it out with repeated solvent wipes. The other side of the wood never goes back to the same colour as the un-contaminated side.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Gee, Bob- couldn't you come up with a tougher challenge?
Drill a large number of holes in each piece. Countersink the holes on one side of each piece. Mix up a good batch of epoxy with high-strength filler. Apply epoxy to the non-countersunk faces of each piece. Clamp with waxed cauls.

Isn't that what is happening on a microsocopic scale when the epoxy fills the very irregular pores/grooves in wood surfaces, then sets to a hard & strong matrix?

I'd also like to see some examples of good adhesion with extremely smooth steel or aluminum (the higher the polish, the better the 'chemical bonding, right?), bonded with common epoxies like WEST/SystemThree, etc. They all require an etching step before bonding metal, as I recall.

Cheers
John

Speaking of burnishing, not sure how it happens, but a properly sharpened and tuned Japanese plane will leave a glossy "burnished" surface, (or at least stripes or areas) on most softer wood. With my plane it is well obvious on spruce, Spanish cedar and sometimes maple, and I can't reproduce it with a metal plane even if equally sharp. The wood looks like it has an extremely thin, well polished lacquer film. I kind of doubt this hampers adhesion, though. I'll have to test one of these days.

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Build log

I feel like you're just trolling at this stage, but I've got 30 seconds:

You can feel free to do the test on a piece of UHMW and a piece of wood prepared the same way and notice the massive gap between the shear and peel strength of the bonds.

And no again on high polish having anything to do with chemical bonding. The etch is to increase surface energy by dissolving the oxide layer and making the surface chemically active. You can chemically etch polished material lightly enough to barely effect the polish, it's how we make anodized things that are shiny, and then you can glue it. The etching isn't digging holes, nor making some sort of structure that the adhesive can 'key' into; surface roughness only aids by increasing surface area. It is possible to etch a fully polished surface enough to increase surface energy to the point where adhesives will work by using plasma etching. With plasma etching we can even glue things to Teflon or diamond-like carbon!

We've gone quite off track, but if that's not enough then you're going to have to start reading for yourself.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Bob, this is great information, which I, for one, really appreciate. In my work, I rely heavily on what I might call "informed intuition". The more information I have, the more complete my understanding and conceptualization of "how things work", the more detailed my mental image of what I'm working with and what I'm doing as I design and build a guitar (or whatever it is I'm making), the more creative flexibility I have in my methodology, the better the final product and the more fun it is to do. I'm not sure if that was a sentence, but I think you get the point.

Still not able to carve out the time to address more of the other points people have brought up. So much to say and do, so little time...

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

I think Todd's idea is quite sound.

Seems quite a few luthiers are going the route of leaving the finish under the outer perimeter of the bridge as a way of getting a perfect look. If done judiciously and the amount left is very small then the penalty of "dead", unglued bridge weight will be likewise small. Normally a tiny rabbet is cut to clear the thickness of the finish. If I understand correctly, Todd is proposing to simplify this by letting the perimeter of the bridge sit directly on the finish with no rabbet. This leaves a void - which is the thickness of the finish - between the bridge bottom and the raw wood we want to adhere to. Epoxy being very cohesive will fill this void and still make a good adhesion of bridge to top. If the epoxy actually adheres to the finish as well, that's a bonus. Either way the bridge will be making contact over its whole area.

Epoxy is great stuff that reliably makes hell for stout joints in wood and sticks to all kinds of other stuff that PVA and HHG are useless on. This should theoretically be moot because we are all going to always acheive micron perfect wood to wood mating surfaces right? Even if we really are, what's the harm if the glue we use is more forgiving of imperfect mating surfaces? To me, making the leap to consider proactively taking advantage of epoxy's properties as Todd suggests is quite reasonable.

I also think there are too many kinds of epoxy and additives to generalize its hardness and damping properties compared to HHG or PVA. Your typical silica thickened West System resin is some pretty tough stuff. There are more structurally oriented fillers even without getting all aerospace exotic. I think epoxy's creep resistance and heat resistance alone make this discussion worthwhile.

Peace,
Sanaka

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...imagine there were no hypothetical situations...

Thanks, Sanaka. You explained it more clearly and succinctly than I did.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Creep resistance, along with repairability is also my concern. Has anyone compared that property among the glues discussed? Creep resistance is a primary reason that many of us have settled on HHG.

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JJ
Napa, CA
http://www.DonohueGuitars.com