What is the knock against polyurethane?

[fscotte]

I don't use it cause I see several dirty rusted cans of it sitting on my garage shelf along with the house paints and deck stain.

OTOH, the quart of Behlen's Stringed Instrument Lacquer peacefully sits in my clean cabinet along side my beautifully designed finger planes and expensive chisels.

[Bernie Daniel]

I don't use it cause I see several dirty rusted cans of it sitting on my garage shelf along with the house paints and deck stain.

OTOH, the quart of Behlen's Stringed Instrument Lacquer peacefully sits in my clean cabinet along side my beautifully designed finger planes and expensive chisels.

Personally I think finishes like varnishes made by secret 300-year old recipes are very cool and I hope those using them never let the tradition die.

OTOH it is hard to beat the convenience of something like lacquer it seems to me.

I don't know where that leaves 2 component polyurethanes as an instrument finish but we used Bona catalyzed polyU on all of our wood flooring last time -- I know it was over 15 years ago and the floors are still in great shape!

[DougC]

I've wondered why spirit varnish is considered a premium feature on mandolins, whereas a more durable and 'waterproof' "nitro" is not. A certain type of 'shine' I guess. I would be in favor of a durable coating.

Also nitro is hardly ever put on violins. (At least Ive never seen one.)

[Tom Haywood]

I used a polyurethane "spar varnish" early on. I think it is a superior finish that does not "kill" tone or looks. What it kills is higher end marketability. Say the words polyurethane finish, and noses turn up and feet walk away. In my experience, different finishes impart different characteristics to the sound and the look of an instrument. Decent ears and eyes will notice. Polyurethane sounds and looks good, IMO. These days, I use only shellac spirit varnish for high end instruments because I think it allows a fine instrument to sound its best, and I use lacquer for cheaper models although it seems that most people prefer the look (and maybe the sound) of lacquer, but I plan to return to the polyurethane spar varnish eventually. It just makes sense.

[peter.coombe]

Ask yourself why Gilchrist, Dudenbostle, Nugget, Smart etc etc all use varnish. I have only ever used nitro on guitars, but switched to varnish (an oil varnish in a water emulsion). Why? Many A/B comparisons were telling me there was a difference in sound. Not huge, but enough to tell me I preferred the sound of the varnish I use. Not just that but customers were telling me they really liked the look of the varnish. What I could hear was a slightly warmer quality and "looseness" more in the varnish finished instruments and I definitely preferred that sound. The difference was small, but consistent. So in that respect I am with Charley Derrington on this. I have also measured the thickness of nitro and the varnish I use and the varnish was less than half the thickness. I also peeled off varnish and nitro from masking tape and I could fold the varnish in half many times and it did not break, but could not do that with nitro. So the varnish is more flexible and softer than nitro, and maybe that explains the difference in sound. I guess it depends on what you prefer. Varnish means a warmer more pleasing tone, but you need to take a little extra care. If you want to tow your instrument behind a truck then varnish won't cut it.

[Marty Jacobson]

"Polyurethane" means very little. It is a very broad term, and basically refers to a part of the polymer structure. Between the polyurethane "links", if you will, you can have all kinds of structures which will much more greatly affect the properties than composition of the uncured resin. The composition of the uncured resin is the only semi-definite thing about the term "polyurethane", but even then there is a broad range of "polyurethanes" on the market, with many different chemical compositions. Most of what's sold as "polyurethane" by Minwax, etc. are only composed of a relatively small percentage of polyurethane, the rest being mineral spirits, "boiled" linseed oil, and other cheaper ingredients.

Polyurethanes can be formulated to be as soft as a rubber band, or as firm and rigid as acrylic, or anywhere in between. I use Enduro-Var a lot, which you could call "polyurethane", but which is functionally identical to a cured alkyd resin varnish when cured (which only takes a few days, as opposed to typical alkyd resin varnishes, which could take months to cure).

I've used shellac, phenolic resin varnish, Enduro-Var, and lacquer. I have not noticed any specific tonal characteristics which I could attribute to the finish used. I always use a very thin coating with any of them. When I used to string instruments up in the white, I didn't notice any significant tonal difference between the unfinished instrument and the finished instrument, whether it was shellac, lacquer, or phenolic resin varnish.

Sure, if you coat your instrument in a soft, rubbery polyurethane, you'd probably have a tone-deadening effect. But given the range of what's possible with polyurethane, I think it's short-sighted to attribute any particular characteristic to the class of resins.

Has it been mentioned yet in this thread that "varnish" is an even more ambiguous term than "polyurethane"?

[Charles E.]

Max Girouard and Andrew Mowry coauthored a review of this product in a recent GAL article..........

https://www.canadianwoodworking.com/...ed-wood-finish

I believe my Girouard is finished with this and it is beautiful.

[Mark Gunter]

Has it been mentioned yet in this thread that "varnish" is an even more ambiguous term than "polyurethane"?

No, but it's a good point. You have to pay attention to know what's being discussed sometimes, a lot of the terminology can be used very broadly. I refer to 'reversible' finishes because most of the finishes we use could be called 'evaporative' finishes ... even the waterbourne polymers, since the water and the suspended solvents all have to evaporate before the chemical crosslinking can occur. But, I can easily understand that when a person refers to an 'evaporative' finish, he/she is probably referring to a reversible finish although I think it's not a good term for that. Likewise, 'curing' and 'drying', etc. You sort of have to be able to read the context when people use these terms in a narrow way.

[pelone]

It seems that a nerve has been struck. I use Poly---I apply it in the thinest of applications and sand between each coat. I have found that when wiping on the poly in the same direction of the grain of the wood and in making sure to only wipe in one direction that it gives a marvelous smooth finish and not only does it dry nicely--it produces a deep luster. The lovely light amber tone that it gives to a blonde instrument seems to bring out the unique characteristics of figured maple without needing to use a base highlighter---the luminescence of the wood seems to shine through. I personally do not like sunburst finishes and feel that the inherent and beauty of the wood of instruments should scream the polite elegance of the wood that has been carefully chosen to backs, necks and tops. Of course--being a contrarian-I often immediately am prone to use a finish just because that is what is "accepted."

[Jeff Mando]

Among Fender electric guitar nuts, of course, nitrocellulose lacquer is what everybody wants, pre-CBS and all that, but so many people today love the late 60's Fenders and even early 70's Fenders -- which are well into the poly-finish era -- that some discussions center around newly invented terms such as "thin poly" meaning "almost" nitro -- which is a crock, IMHO -- since you can easily tell nitro from poly at a glance....the thinking was that at first Fender didn't lay on the poly so thick. I think of my old 1975 tele custom with the maple neck and the fingerboard finish was almost as "tall" as the frets! Those guitars played like they needed a fret job when they were BRAND NEW! Not a good era for quality. But, a great sounding guitar, Keith Richards and all that stuff, so I had the frets removed, fingerboard sanded to bare wood and refretted with "Gibson" frets = a very playable guitar!

[multidon]

This has been a fascinating discussion! Thank you for all the contributions. Some observations on my part:

1. The term “poly” is confusing to many. When you use the term “poly” does it refer to polyurethane, or polyester? Fender uses the term “urethane” to refer to polyurethane and “poly” to refer to polyester.
2. Fender did use nitro in the 50’s and 60’s, but not exclusively. Sometimes, acrylic lacquer was also used. Many of their finishes were straight off the shelf automotive paint. Some of the color names, like “fiesta red” and “arctic white” are unchanged from the automotive terminology. Modern American Fenders are mostly urethane, except the Vintage series is nitro. Mexican Fenders are polyester.
3. The musical instrument industry quickly switched from varnish to lacquer (acrylic and nitro) simply because the sprayability and faster curing time made production much easier than using varnish. The decision had nothing whatsoever to do with the sound quality.
4. Breedlove used polyurethane on their USA mandolins and they are well regarded by some.
5. I have not seen many references to scientific studies regarding the effect of finish on tone. One such study, concerning violin varnishes, was done by a fellow named Cremer, and his conclusions were presented in a work entitled “The Physics of the Violin”. Cremer states the case very simply. “Varnish has one virtue; it is very thin”. James Beaumont, in his book “The Violin Explsined”, published by Oxford University Press, states “Provided the varnish is neither thick,nor glass hard, nor rubbery, it cannot have any detectable effect on what is heard from an instrument”. He further states “if a physicist can measure some change in the sound from an isolated instrument, which can be proved to be independent of unstringing it, the time involved in varnishing it, and restringing it including resetting the soundpost, I will be impressed”.
5. This notion of certain finishes being “open pore” or “allowing the wood to breathe “ has always puzzled me greatly. If wood has any need to breathe at all, which I doubt, doesn’t it breathe just as well by virtue of the inside being completely unfinished?

There appears to be little scientific research on this subject, and those that exist come down on the side of “it really doesn’t matter, as long as it’s thin”. Why don’t we all just play instruments in the white? Because they would be ugly and soon show dirt and grime which would be embedded and impossible to clean off. So we finish instruments at least partially as an aesthetic and practical matter.

It would seem that a lot of the notions concerning the effect of finish on tone held by numerous musicians consist of belief, not scientifically confirmsble fact. A case of “I can’t prove it, but I know what I hear”. But in his book, Mr. Beument points out the “exraordinary susceptibility of our hearing to suggestion”. So some of these preconceived notions will die hard. A truly scientific study of this subject would, in my opinion, be an interesting exercise but extremely difficult to execute.

[ProfChris]

Another reason why we don't play instruments in the white is that many builders assert that they can hear a change in sound, for the better, once the instrument is appropriately finished with any one of the commonly used finishes. I can detect this in my own builds (or think I can) - it's as if the wilder overtones are tamed, rather like applying a little compression to a sound recording. I've read suggestions that this is caused by the shrinking of the finish, but that might be sympathetic magic thinking (shrinking=compression=reducing overtones).

All I can pull out of the mass of anecdotes and opinions is:

1. Builders generally think that any of the common finishes can produce a good result in terms of sound.

2. Each finish is different in terms of application technique and repairability, and also in appearance - these are what leads builders to choose between them.

3. When building for sale, the market perception of finishes is a major factor in choice of finish. I've come across amateur-built instruments completely finished with CA glue, levelled and buffed, and this can work fine but it would be a hard sell!

[gweetarpicker]

I think the knock on poly finish is more a matter of perception. Poly gets a bad reputation from the usually (but not always) cheaper instruments you see that are entombed in a thick plastic sheet. However, if properly applied, poly finish can be a thin as nitro. Poly finishes are more stable and less susceptible to degradation but are typically harder to repair. My repair guy peeled this 15 mil thick sheet of plastic off the back of a customer's $2,000 boutique built guitar ( I won't mention the brand) that he had in his shop. This one piece of peeled finish weighed about 2 oz so we figured the guitar had a 1/3 to as much as a 1/2 pound of finish. The repairman went back with a much thinner nitro finish, and, at least to my ear, the tone was improved.

On the other hand, Taylor, Tacoma and others have made many great sounding, beautiful guitars with appropriately thin poly finishes over the years.

www.vintagefrettedinstruments.com

[multidon]

But on the plus side, at least it was nicely bookmatched!

[sblock]

Most of the important point have been made. Here are some of them:

1) Finishes are mainly there to protect the instrument, and to add aesthetic appeal.

2) Finishes do not serve as buffers against humidity, because the inside of the instrument is unfinished.

3) The thinnest possible finishes are considered desirable.

4) Technical points: A thin finish therefore adds negligible mass. A thin finish can, however, perturb the stiffness, but only very slightly, affecting only the highest overtones, which are those that involve very short length scales (this, related to the elasticity of the film, once cured). A thin finish can also serve as an additional source of damping. But again, it is not the dominant source of damping, by any means! For these reasons, a mandolin in the white can sound slightly different from the finished version, with a more "brittle" (less "warm") tone, likely due to the increased amplitudes of some high overtones. Many luthiers report hearing a small difference between mandolins in the white and when they are finished. The usually consider the finished tone to be "improved" over the tone in the white, possibly because it is less brittle, or possibly because it more closely resembles the paradigmatic model of what they were going for, which was a finished mandolin (e.g., a Loar-signed F5).

5) There would seem to be is no particular reason why a varnish, nitrocellulose, or catalyzed polyurethane finish might not fulfill all the criteria, above. Yes, the cured films do have slightly different elastic (stiffness) properties, but only slightly. I very much doubt this would amount to much, because nitro and poly can be formulated in ways that make them less brittle, and more akin to varnish. However, most of the comparisons being made are comparing apples and oranges (!), because they're comparing films of different thicknesses -- not comparable thickness. And too thick a finish will definitely kill tone. Or, they are comparing a brittle finish to a pliable one. Many inexpensive PacRim imports, for example, have excessive finish on them. It's much easier to cover blemishes and surface irregularities with a thick finish, so this greatly speeds and simplifies production steps.

6) Finally, let us not forget that most musicians (and many luthiers, who cater to these customers) are very conservative, as a whole. If the best-reputed mandolins in the past had a varnish finish (e.g. Loar F5's), then they want a varnish finish on their own instrument! They may also prefer the look, although it is possible to very nearly match the look of varnish using other different finishes, so this is not really the driver. Musicians simply know that some of the greatest mandolins have been made with varnish, and so they want it on their mandolin, too. And this makes a certain amount of sense. But it does tend to inhibit faster development of alternatives.

From a materials/physics of music perspective, I see no reason why a finish other than varnish might not work perfectly well. However, great care would have to be take to get an equivalent thickness and cured elasticity (stiffness). But this is certainly achievable with modern finishes. Such a finish might not be much more durable than varnish (after all, it would be just as thin and pliable as varnish), but it might be more reproducible, and much easier -- and faster -- to apply and cure. Some varnishes can take months to years to settle down, after all, and that is not good.