Official Luthiers Forum!

I was going to ask this question in the thread on French Polishing, but I didn't want to go off on too far of a tangent.

Here are some parameters for the question:

First, assume that I'm asking this question in the context of the finish material choices being limited to the normal pallet of finishing materials that guitar makers use (shellac, nitro, more modern finishes, etc.). No need to address ridiculous choices, like motor oil or mayonnaise.

Second, assume that we all understand that film thickness has a significant impact on tone. Thinner finishes inhibit the tone less than thicker finishes. Some finishes go on so thick that they really inhibit the finish in a noticeable way. This question is not about film thickness.

Third, this question is not about any other attributes of the various finish materials, such as ease of application, durability, repairability, environmental impact, etc. Those are all important factors, too, but this question is about tone and only tone.

The question:

Assuming one can apply an appropriate film thickness (whatever that is), does the choice of finish material affect the tone of the guitar? And by affect, I don't mean in an abstract, "everything has an impact" kind of way. I mean in an "I can definitely hear the difference" kind of way. Again, with emphasis: This is assuming an appropriate film thickness.

You hear folks say things like "shellac is the best sounding finish." I suspect that such statements are more about the inherently thin film thickness one gets by French Polishing a guitar, which is really only done with shellac and modified forms of shellac. So, I understand the blurring of material and application method that happens when we talk about shellac and French Polishing. But it does make it harder to discern what is really happening.

I think I remember position statements on this issue in the Gore/Gilet and Cumpiano/Natelson books, and probably a few others. But I'm interested in what OLF members think, and why you think it.

So, independent of the other attributes of the various finishes we use, and assuming an appropriate film thickness, does the choice of finish material (assuming the normal list of choices) affect the tone of a guitar?

Here's a fun and simple test. Take a piece of plastic and tap it to hear the ring tone. It sounds just like ... um.. plastic. An awful cardboard sound that is completely dead. I would not want sound deadening plastic encompassing one of my guitars. So I don't use urethane, poly, or water based lacquers which are essentially plastic. They can not possibly help the sound.

That leaves shellac and nitro. I'm highly allergic to lacquer thinner, so I use shellac on my instruments. Both are brittle finishes that need to be applied thinly. Both have solvent fusing between coats so no witness patterns are seen if sanding is needed. Win win.

Hi Don,
I've thought about this quite a bit and will give you some of my thoughts on the topic.

I've always outsourced my finishes. I first used Tony Ferguson (urethane finish) and then switched to Joe White (polyester). I've also had nitro applied to my guitars as well. I started out with a traditional French Polish finish for my 1st 10 or so guitars.
Then I tried an Adam Stark Nitro finish. I definitely noticed some loss of tone. Then I used Tony for a few years. Same deal - my guitars lacked some vibrancy but my guitars were also improving so the finish effect was not as relevant.
Then I used Joe White for the remained of my building career until recently when he stopped doing finish work.
During this time I did a few guitars with FP tops and polyester everywhere else - these guitars were always remarkably vibrant and came out of the gate with a sound that seemed decades old.

Here's what I've noticed. When I French Polish the tops, I do not notice a difference between pre-finish and post-finish of the body. I determine this by tapping on the body and listening to its response. I also don't notice a difference between pre and post with the oil varnish I'm currently using either. I always noticed a difference with every other finish type that I've used.

I have some questions as to why this is and I'm thinking that some of the loss of tone is due to film thickness. My FP is quite thin as is my oil varnish on the top - definitely under 0.003". Tony's finish was definitely thicker than that - maybe 0.005" or so. Joe's finish usually came in at around 0.003" or so at the bridge but it was often thicker at the fretboard region. I measured the finish with calipers after chiseling it off to place my bridge/fretboard. My gut instinct is that Joe's finish probably varied in thickness a few thousandths - perhaps the bridge was thinner and it was thicker in other places - I don't know.

I believe that with spraying these finishes, it can be harder to maintain a perfectly even finish over the top - some parts will be a bit thicker than others. I think oil varnish might be a bit easier to spray ultra thin but I might be wrong (I use a brush and can very accurately determine how much finish I'm laying down with each coat). Maybe this accounts for the difference in tone or maybe it's the material, I honestly don't know.

French Polish is much more difficult to build up - so almost everybody that uses this finish will get an ultra-thin finish. Oil varnish seems to be a bit similar - I think most folks will end up with a thin finish assuming they don't put on more coats than they need. But even so, I'm quite sure that at least a few of the guitars I got from Joe White would have had very thin finishes everywhere - and they still never sounded as vibrant as the shellac or varnish. This leads me to believe that even with an ultra thin 0.003 finish, the coating materials of shellac and the resin/oil blend in varnish sound better than polyester or urethane. I don't have any recent experience with nitro - so maybe a thin nitro finish would sound close to the shellac and oil varnish - that's something I might explore in the future - although there are a lot of things I don't like about nitro and so I'll probably just stick with the oil varnish.

Interestingly enough, many oil varnishes use a resin that is basically polyurethane (urethane modified alkyd). Oil varnishes with pure alkyd resin are basically using a polyester resin. The dynamic with the varnish is that the resin is heated with the oil (tung, soy, etc ..) and you end up with a "blend" rather than just a mixture. So the heating creates something new. The resulting oil varnish films tend to be quite flexible and so the final finish is quite different from a urethane finish that's on a car or the polyester used by Taylor.

I don't want my guitars to sound the way they do without finish. I like the slight damping quality of the sprayed nitro I use, it seems to sweeten the tone and eliminate the harshness.

Hi Jshelton,
Have you strung up a guitar before putting a finish on the top to verify that you prefer the tone after you've finished it? I say this because it seems counter to my experience. Guitars that come back less alive (and I've built enough instruments to draw conclusions from) always sounded a certain way. Compared to the few guitars that were French Polished and lost none of their vibrancy and resonance, these instrument always had a "played in" sound. With both urethane and polyester top coats, the increased damping yielded guitars that were harsher sounding and less warm out of the gate. In short, your conclusions seem opposite to mine - not that yours are wrong. Additionally, nitro is one of the least damping finish coatings. If you are applying a thin finish, it is quite possible for your finish to be having a minimal effect on the tone (something I would say is a good thing).

Without having done a controlled study, I would say yes.

There has been a distinction between my normal finish and my gloss contracted finishes, that is not consistent from material to material. So while all my normal guitars sound one way, the gloss guitars sound different, but not the same different, if that makes sense.

I don't think you can divorce the material from film thickness, as each material will have its own 'appropriate film thickness.'

However, as a thought experiment, let's say you had a magic guitar. And on this guitar, you could magically swap out different finishes of exactly the same film thickness. I believe you would hear a consistency to the difference in tone that each finish made, as they each have differences in weight, damping, and flexibility.

The closest you could get to finding out would be to build a batch of similar guitars, string them in the white, record it, get them finished, and rerecord them noting the changes. But you'd have to have multiples of the same finish so that you could hear the 'flavor' of each finish over more than just one guitar. So say you did this with 20 guitars, had four done in nitro, four urethane, four poly, four endurovar, and four shellac, or whatever finishes you want to know about, then you'd have a reasonable baseline for basing a judgement. But even then it could only be suggestive, as you'd need your magic guitar where the only thing to change would be the finish to make it properly controlled. And who's ever going to do that?

I also believe that your guitar has to be in a range of sensitivity for finishes to make a clear difference. If your top is overbuilt, the finish will make up less of its overall mass, and it's effect will make less difference.

I think in the Gore/Gilet books it says something along the lines of 'When all else is the same, look to the finish'. And specifically recommends staying away from polyester, which I would agree with as it's consistently been the finish that has sucked the most life from my guitars. I won't use it again unless Joe White gets back into the game and I can get one done at .003, but that's just me ticking off a 'what about' box...

Blah blah, sorry...

First you must understand that type of material and film thickness are linked in many ways through molecular bond strength and weight. Different finishes also have different application weights specified for normal function. For instance you can get a FP to function and survive at 2 mils but a nitro won't even give good coverage at those final weights. Then there is how stiff the dried film is. A varnish will be softer than a modern acrylic.

Back in his tenure with Gibson, Roger H. Siminoff did a controlled study on nitro lacquer thickness versus change in tone. The study is detailed in "the Lutrhier's Hanbook , by him, published by Hal Leonard. Chapter 10 deals entirely with this study and it's conclusions. But some highlights are adding mass lowers fundamental but the increase in stiffness as the coating gets thicker raises it, so this becomes mostly a null effect in the normal range of final thicknesses. The effect of increased mass is most prevalent in the first 4-6 coats with increased stiffness become more prevalent as more finish was added. Adding finish always increases the amplitude making the part sound brighter.


So weight (mass) is likely the biggest thing the instrument will "see" and change in stiffness the second. As these two parameters change on your top so will your voice. Mass is more important because stiffness comes on with thickness and we are striving to keep finishes as thin as functionally possible.

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Now back to the original question. Does type of finish make a difference? to some extent yes, as defined by the parameters above. But it is not a simple comparison by any means..... Actual molecular weight of the final film as well as MOE in detail at very thin sections would need to be absolutely known to make comparison between any systems.

And if one had that data they could actually plot a final film thickness between different finishes that would have the exact same tonal impact.

Now my question, how do you intend to apply, level and buff everything you ever do within a window of +/-10%? At 5 mils target the window is .0045"-.0055". As someone who has finished countless guitars..... I doubt you could hit that reliably with computer controlled air assist spray systems. Especially when one considers guitars have curved surfaces. And is 10% a tight enough target?

Second, do you really believe the magic sauce of tone resides in 5 mills of resin applied only to the outside of the instrument? If this was the case we would have settled on varnish or shellac long ago for everything. Doesn't it make more sense that inconsistencies in tone actual generate from other build issues like uniformity of voicing?

Then finally, at most there are 18 grams of finish on a dread top. Maybe 12 of which actually reside in a place on the top that makes sound actively. Yes that is a good deal perhaps. But do you weigh everything else? bridgeplate, bridge, pins, braces, tone bars, top, rosette etc? how much variance do you see in weight here on your voiced tops? yet it must be the finish, right?

Those prized vintage martins and Gibby's.... you'd be amazed at how thick some of those lacquer jobs are! 12-15 mils and more I have seen. Yet these are the holy grail of tone? It's not the finish!

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

On some of the early romantic guitars and lutes the backs and sides were varnished and the soundboards were left unfinished (or possibly given a light coat of glair).
Instruments with extremely light tops (lattice braced, double tops) might benefit from a finish that would add some mass to make them sound a little less "banjoey".
As the availability of solvent based finishes "dry up" it will be harder to source finishes with the look, ease of use, and serviceability of some of the traditional finishes we presently use.

It's a good question and good discussion. I don't feel experienced enough to make any kind of judgement. But anecdotally for me personally when I started French Polishing my instruments sounded better to me. Of course I started FP after about 50 guitars so I got better at making them too. And my only comparison comes from using Nitro, Water based stuff like KTM and a few in oil varnish. One of my very best sounding guitars was with water base but years later it looks absolutely terrible. Fortunately the owner loves it so much he doesn't care. But the point I am trying to make is it seems this question is impossible to answer. We all have our own personal experiences and even when they jibe with others there is no real data to suggest anything.

In the end I look at it like this. Based on my experience (and after all that's why clients come to us right? They trust you based on your own personal experience) I recommend FP for optimal tone. But in the end if a player needs wear resistance then I will do Nitro or farm it out to someone at their expense for a UV finish and the difference in tone if even perceivable doesn't outweigh that practicality of a stronger finish.

I always play the guitars before finishing. It allows me to make alterations to the top thickness and taper which is required almost every time. Take into account though that we only build classic and flamenco guitars so the tone we're after could be entirely different from you want. We've been doing it this way for over 50 years.

Thanks to all for participating. Two things that I hope can aid discussion:

First, assume that by "appropriate film thickness" I am referring to whatever is appropriate for that finish material, but keeping it as thin as reasonably possible for that material. I realize that this would make it difficult to do a direct comparison at identical film thicknesses, but direct comparisons are impossible, anyway, since no two guitars are the same. We do our best with what we have, right? Also, while film thickness varies, it tends to be within an overlapping range for most of the materials we use, right?

Second, Brian has addressed a straw man that I want to eliminate: the idea (which I didn't and don't promote) that finish is the be all and end all of guitar tone. I don't think that there is any secret sauce in a finish that either makes or breaks the guitar. There are far more important factors in the tone of guitars, as Brian states. This is a little like microphone or preamp choice in the recording world; there's no "talent" button on a microphone. In like manner, a great finish is not going to make a mediocre guitar sound good. At most, a bad finish can inhibit the otherwise good tone of a good guitar.

So, let's assume the finish we are discussing is on an otherwise great guitar. Assume the finish is not the most important factor for tone. Assuming an ideal film thickness for each finishing material, and limiting the finish material choices to our normal choices, do some finish materials significantly hurt tone more than others?

Common statements we hear: Shellac is the best sounding finish material for tone, because it doesn't hurt the tone that is already there. Polyester finishes suck the tone out of a guitar. It feels to me like some of those views come from reactions to film thickness differences. But maybe it is also about the material itself. That's what I'm trying to get a feel for. To the extent the finish plays a role in hurting the tone of a guitar, is that just about film thickness, or is it also about the finish material chosen? And if the finish material is a significant factor, how significant?

Considering the fact that people buy guitars at all price levels with all the commonly used finishes I would say the answer to your question is "insignificant". Like the materials the guitars are made of you can argue the virtues and failings of each. Martin sells guitars made of HPL and others where they use traditional materials and hide glue.
One other "component" that is often overlooked is the person playing the guitar. What makes a great sounding acoustic blues guitar - that it is ladder braced and cheaply made, or that blues players played ladder braced cheaply made guitars?
Some finishes are better than others for some things, but more importantly, some finishers are better with some finishes than others.
Finding one you are happy with and getting good at using it is more important than finding the ultimate finish.

This is kind of an aside to this thread but... When I first started building I built classical guitars and tested them in the white. Just curious, do you finish with the bridge on? Or do you remove it after testing? I always had a heck of a time finishing with the bridge on so I don't do it anymore.



I think I disagree with that but have to explain. I have found that a finish of any kind does at lease *somehting to the tone of the instrument. I might not be able to put that something into words but I've often said things like "tightens it up" or "brightens it up." And so on. So the tone that is already there in the white changes no matter what finish you put on it for better or worse...

As to the rest of it, I can't remember which very well respected luthier it was that said something like, epoxy is good for a boat but I would not want it on my guitar... I guess even if the epoxy is the same thickness as a FP film

What you have there is a knee jerk response based on what you said the finish was and that persons perceptions of it.....I have applied FP, Nitro, 2K urethane, catalyst polyesters, UV urethane, UV poly, Amino alkyd... and I'm sure at least one other I have forgotten. Assuming your in the "sweet spot" for each thickness you will never hear any difference. Avoiding this reaction is why most factories do not even mention what type finish they use.... typically using a generic term like "varnish" if anything.

I deflection tune all my tops so they are as mechanically uniform as possible. And side by side by side, FP vs. post cat poly vs. UV poly you will never hear anything that you can attribute to differences in finish either by material or weight assuming they were all applied at the sweet spot for their type.

Lets take Polyester for example as everyone like to beat up on it.... It gets a bad rap from cheap asian instruments with finishes 1/8" thick. but if you actually look into it you will find that UV poly isn't all the same or anywhere near equal in molecular weight per cured mil or modulus of elasticity. And while you can apply the stuff an inch thick, the ones better suited to our needs can live at 2 mils.... and will survive better than a 2 mil thick FP. They can be, and are, engineered with specific gravities and final MOE to provide just the type of performance we are looking for. So is it a tone suck? Ask Taylor..... they sell an awful lot of instruments, and while their tone is very take it or leave it, they sound nice and bright and very consistent despite having UV polyester finishes at about 9 mils up until a few years ago... new ones seem to be 5-7 mils. Taylor's formula was specifically engineered for them by an aerospace manufacturer to achieve this. So people have delved into this topic much deeper than we ever can and have still not came up with the perfect guitar finish that will replace everything else the instant you hear it....

This whole thing to me is the same rabbit hole as the hot hide glue verses alaphatic resin thing..... Your time is better spent improving your builds in other ways. Like for instance have you noticed the effect shell border has on tone by stiffening the edge of the top? There is something to be learned there that you can actually control...

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Thanks, Brian. Your conclusions in your last post are the ones I would come to, but I do not have the experience level that you (or others) have, so I wanted to ask.

Finish with the bridge on and yes it's always a challenge to get the finish to look good at the perimeter of the bridge. The bridges are filled (Z-Poxy) and finished before they're glued on. We just went through the finishing room with vacuum and mop to prepare for spraying a batch of three flamenco guitars.

[quote=
The question:

Assuming one can apply an appropriate film thickness (whatever that is), does the choice of finish material affect the tone of the guitar? And by affect, I don't mean in an abstract, "everything has an impact" kind of way. I mean in an "I can definitely hear the difference" kind of way. Again, with emphasis: This is assuming an appropriate film thickness.
[/quote]

Violin makers have done many rigorous tests of the acoustic impact of various finishing materials. One of the better studies can be found here https://stacks.stanford.edu/file/druid: ... kq5616.pdf (The article begins on page 27). The author of this article summarizes his findings on his website:

"Numerous measurements made in the MARTIN SCHLESKE MASTER STUDIO FOR VIOLINMAKING have revealed large differences in the damping behavior of different wood treatments using primer and varnish. The spread is between 0.75 and 3.0 compared to untreated wood. In other words, some formulations produce an undamping (!) of the wood by up to 25%, whereas other substances triple the damping of the wood (!)"

The testing is done on strips of wood so that changes in damping and speed of sound can be accurately measured. On a finished instrument there are so many other factors contributing to the sound that it might be difficult to be sure you are hearing the effect of the finish. You could say the same thing about the selection of woods, the back thickness, or the bridge weight--things that most builders try to control.

Ok devils advocate here then. Why can't you dent the top with your finger nail on a UV-Poly finished instrument but you can on an FP one? That has no bearing on tone?

Why would it?

I find these differences of opinion so interesting.

I beat up on poly not because of Chinese instruments with a 1/8" topcoat, but because of what it does to my guitars with a standard 5-7 mil coat. It steps on the trebles as clear as turning down the treble knob on an amp.

It's not subtle or indistinct, it's epic and injuring. So I find it fascinating that some folks find it brightens up a guitar. One of my more math inclined comrades found it raised to top freq by 5hz, which is a pretty big shift.

While I've certainly not gloss finished as many guitars as Brian, I've built +250 over the last 7 years so I feel I have a good baseline for judging what my standard guitars sound like as a class, and therefore a good baseline for what other finishes do in comparison, and again, the differences aren't subtle but distinct, all largely based on how much they step on the trebles.

That said, there's lots of folks building fine sounding guitars with poly. I guess like all things guitar, since the finish couples with the wood and becomes part of the system, it's hard to isolate the effects.

But I'll double down that I find the finish makes a huge diffence in my guitars. I guess YMMV.

Because we are talking about the physical properties of the finish. One is apparently a lot harder then the other. I would think that the physical properties of the finish would have something to do with it. How could it not?

This is a really good discussion and I've enjoyed reading everyone contributions, especially Brian's as he has a lot of experience on this topic.

I'd like to emphasize one of my earlier comments but frame it with a couple of caveats:

1) I'm building steel string guitars
2) My guitars are ultra resonant/alive/responsive. In comparison, most Taylors/Martins/etc. sound like they have a stock stuffed in them.
3) I am incredibly consistent from guitar to guitar.

On my instruments (just the body), there is a perceptible decrease in the sustain and resonance of the body after it has been sprayed with polyester (Joe White) or urethane (Tony Ferguson). Joe's finishes were fairly thin (definitely under 5 mils) and Tonys were probably around the 5 to 6 mil range. This change didn't cause the guitar to go from good to bad. I've been using Joe White for more than a decade now and my guitars would still be considered incredibly vibrant/responsive instruments.

That said, the few times I've FP'd in recent years, those guitars were perceptibly more resonant, vibrant and the trebles were smoother, less harsh and the bass more open and warm right out of the gate. I build in batches so I have been able to make excellent comparisons. So for example, two spruce/rosewood guitars that have bodies that sound near identical (at least, in terms of resonance and sustain). One goes off for a poly finish and the other gets a FP. Believe me, there's a difference and one that I've had non-playing family members easily observe.

The difference iisn't massive - it was a subtle difference but it was there. Again, oil varnish does the same thing for my instruments - almost no difference from pre to post-finish. So for me, no question, I get better sounding guitars with FP or oil varnish. Not massively better but subtly better. I have yet to do a nitro finish in recent years - so I can't draw a conclusion about it.

I still think that a thin polyester finish is an ultra-nice finish - but tonally, I believe it restricts the potential of my guitars. I also think it is possible that some builders may find that the damping of the polyester, urethane, etc .. helps their guitars.

Addam finished a lot of guitars in nitro for me before I switched to Tony. Addam did a great job and I was concerned about a change in tone with catalyzed urethane. Turned out I couldn’t hear any and neither could anyone at the store where I sold. Tony is excellent.

Thickness at the bridge was around 3-4 thou on both, interestingly a little thicker towards the ends of the wings.

I have come to like catalyzed urethane not just for the way it looks but the ease of clean up on fretboard edges, nut slots, and final flossing of the heel fit. Seemed like I would frequently get some little chips or lifts with nitro that I had to repair.

Oil varnish finishes really interest me. I have never played one with that kind of finish. Are you doing your own now Toonces?

Tone is fascinating, especially subtle changes. I think we each hear a guitar a little differently and the sound goes through a lot of filters before it hits the end zone not the least of which is preconceived opinions on different finishing products.

_________________
It's not what you don't know that hurts you, it's what you do know that's wrong.

Typically because of difference in film thickness. The FP is much thinner than the typical polyester. But rest assured at equal thickness they behave much the same in this regard. Under about 5 mils the softness of the spruce underneath is the limiting factor. Why I wear mechanics gloves when I buff, to avoid fingernail marks.....

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/