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Does anyone out there put a coat of shellac on the inside of they're guitars? I noted that both Ervin Somoygi and Mike Greenfield do this.
In Somoygi's book he states that it helps protect from moisture and as it is one seal coat only, he doesn't believe it has any affect on the performance.
I did this on the body I just closed up. Curious if any one else does this.


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It/s a pita for repairpeople..I did it once , and it seems, IMHo to have little real benefit ?. Better to hear from someone who uses shellac

it's on the list of things I'm going to do on my next build, just to make the interior look a little better. I doubt a wash coat will do much to protect it though, and shellac isn't much protection against moisture anyway (try setting a damp glass on a shellac-finished table to a minute)

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I do it to the back of the instrument, just because I like the look of it from the soundhole. Just a couple of coats.

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It's a poor practice in my experience.....

Two reasons why one might do this, three actually....

1) Slow the absorption of excess moisture into the wood when the guitar encounters high RH and conversely to belay the guitar drying out in dry conditions.

If you conduct your own testing about permeation of moisture through one or two coats of shellac or many for that matter we speak of hours.... not days.... not months..... not years.... With this said reason one is a myth.....

2) Looks.... Some people like the look, I don't.

3) As Ernie said repair people don't buy into this and the reason why is we often have to work blind with our arm in the inside of a guitar to reglue a brace either because of damage, owner negligence or builder poor glue practices.... Shellac makes it more difficult for our glues to grab hold and do their job.

I'll add that I actually think it looks like *&*( and if you are super anal as I am it adds weight, only a gram or two and who's counting? I was when I was building, I counted every gram.....

When wondering what to do ask yourself "what's the harm." In this case for my newly coined term, "serviceability quotient" using shellac on the inside of the box reduces one's serviceability quotient..... making it something that I would never do.

And lastly for the broken record here.... Guitars are ultimately tools for musicians, not another pretty face, not just a woodworking project, etc. Build them to be serviceable and you won't get flamed in that regard on the Internet someday.... by a pissed off person who bought one of yours second hand off the "used" rack....

I've only ever done it on Selmer guitars as is part of the tradition. But it seems to make sense to me. It will help, if only minimally but still, the moisture issues. That's just logic, take two boards one with a coat of shellac and one without, which one will defend itself better from moisture? I also don't get, but always hear when these threads come up, the repair problem? If a brace cracks off at the glue line you are going to glue it back down to its original location any way right? So the shellac should not be a problem there. It could be a problem if you need to cleat a crack but then you can can use CA for cleats too.

I've always thought about it but I never do it. It's just another step and for what value you may get from it is minimal or as others suggest possibly disadvantageous. So I don't bother.

On the topic of shellac as a vapor barrier:

Gupta, S, John, A., Kishan Kumar, V.S. 2016. Studies on effect of coat thickness on the moisture uptake by a hardwood substrate. Maderas, Cienc. tecnol. vol.18 no.3

http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0718-221X2016000300005

An interesting recently published paper that suggests the following:

- Even thick shellac coatings of 0.002"/.05mm have minimal effect on the rate of vapor transmission or final moisture content of hardwoods (and I suspect the softwoods we use for most tops)

- Thinner films are noticeably less effective, with thicknesses much less than what we commonly see in fresh french-polished exteriors of little benefit in reducing vapor transfer or controlling moisture content

It appears to me that - given these results - interior finishing is a matter of appearance, rather than function, and - at least in this shop - the disadvantages of dealing with interior repairs offsets any aesthetic impact.

A few months back, the topic of interior finish and appearance came up when repairing a small shop custom classical. To say the low RH damage repairs were made more a challenge by the heavy shellac interior finish would be an understatement, but the fully french-polished center graft was a very distinctive touch. One of the boys in the shop (also a talented cabinetmaker and timber framer) wryly suggested that - if I preferred heavily lacquered traditional Chinese furniture - I might like to explore that finish on my own project, while if I preferred the silky feel and subtle, satin appearance of the planed, unfinished surfaces and visible joinery of Japanese tea houses, that same look might be achieved inside the box with diligent fitting, scraping, and a bit of sandpaper. Certainly something to think about.

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Finishing the inside is recommended by Bob Benedetto in his masterpiece on archtop guitar construction. The archtop guitar design is such that opening the box for repair is a very uncommon occurrence. I started building after having read Bob's work so I thought all instruments had this, so I used it on my mandolin builds as well as archtop and flat top guitars. They say old habits die hard and that describes me well. I have sealed the inside of every build since 1999 and so far so good. Matt

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I do it, always have. I build Selmer style guitars which have arched tops that move up and down with RH changes, this changes the string action. I doubt the thin shellac coating will ultimately stop moisture in/out, but I'm convinced it slows it down, which is helpful. I've seen Selmer style guitars with top not sealed inside go sharp 50 cents in a single set when subjected to a significant RH change.

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That's very interesting but still the tests were over 34 days and in all cases it certainly shows better results then uncoated. I think luthiers are more concerned about moisture uptake in hours and over the period of a day. I also wonder about the difference in that study between what they used, cubes of wood, verses panels that we use. Could perhaps almost all that uptake occurred in the exposed end grain?

A couple of years ago, John Monteleone displayed his 4 Seasons guitars at the Woodstock show. I chatted with him and he said that he always shellaced the interior of his guitars. These 4 guitars had a lot of inlays on the INSIDE that could be seen through side sound holes, and you could easily tell they were finished inside.

Ed

I don't dispute that there may be some benefit to slowing down the rate of vapor exchange, but it would appear that to achieve that benefit, the film thickness might be much greater than builders are using. I find it interesting that the film thicknesses required were so great - the notion that a coat or two on the interior would provide a meaningful thickness seems optimistic.

Cubes versus panels is an interesting question - the cube would represent a worst case if fully sealed (as the paper suggests) - much less surface area per unit volume versus a thin panel, so much slower vapor transmission overall. Certainly unfinished end grain sees faster transfer, but also deeper penetration of sealers, so I wonder whether we'd see much change in the finished samples had the end-grain been minimized?

The most pertinent portion of the results appears to me to be that there was not much change between the uncoated samples and the samples with the thinnest coatings (still about .002"...heavy for a guitar coating). Keep in mind that even the thinnest coating shows just .6% or so weight change at 72 hours as compared to unfinished...which begs the question on what the maximum feasible film thickness might be on the interior, and would that film thickness be any improvement over an unsealed interior? Or - to restate - is there a better reason for doing something than custom or subjective judgement?

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I put a coat of shellac on the inside of an uke that I shipped out the end of last year. Mainly because I knew that it would be played outside a lot and felt the 1 coat of shellac would slow down the effects of humidity changes. Hopefully with the little extra protection it will be a plus in the long run. The added benefit was that it made the color of the Koa pop a bit on the inside.

Sealing the inside is a way to help to prevent sudden moisture loss from occurring as much as it is for preventing moisture absorption. It is the sudden drying of the plates that can often cause problems like cracks. Sealing the inside of a carved instrument has a greater effect than with a flattop, in part due to the plates carved areas that expose more end grain where moisture can be exchanged. That could be one reason that sealing the inside seems to be more common among archtop builders.

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I have heard not to finish the inside of the top. The other areas seem to be optional.

My Kohno 30 is finished on the inside of the back. I am not sure about the sides.

It's usually pretty easy to spot humidification abuse, both ways. My instruments were built at 42-45% and everyone that bought one knew it. They also knew that once it left the shop, it was their problem.
Here's something else to consider. Even a thin coat of shellac will change the vibratory characteristics of the box. On top of all that, a coat on the inside restricts the evaporation of the solvent in the finish on the outside of the instrument. Once the finish is applied, the only evaporative opening is the sound hole. Yea, yea, OK, and the end pin hole...

The build that I shellacked the interior of is a OLF SJ. E.I. Rosewood. Engleman top.
I had recently got one of my builds back from a second owner. The guitar was out west in Colorado. I went online and checked the annual weather and discovered that the humidity gets above 50% only 6-8 weeks each year. This guitar had dried out quite a bit. I was able to re humidify it and successfully repaired it. But I'm certain that a shellacked interior in this case would have had no beneficial effect.
My thinking was to slow down the ( all my builds are domed tops) lifting and falling of the top for short playing sessions. An hour or two. But I can definitely see that to have a greater impact in this would require a much thicker coat which would also have a detrimental impact on the tops tone.
So perhaps I should ask Mr. Somoygi or Mr. Greenfield?


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I do it mostly because it seems to help keep it clean in there during the build. But I put a minimal coat on. Not sure the lay person would even notice.


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Dave and I have an expression that we often say in our shop regarding builders who don't respect the idea of serviceability.

Do what ever you want, we don't care, we'll just make more money fixing it and not be shy to explain why to our mutual clients....

Good day!

A wash coat of HHG on the inside. That way you have both sides of the coin covered.



Just kidding.

I still don't understand how a shellacked inside would hamper regluing a brace back down in the exact same spot... As I mentioned before you can use CA for cleats and a cracked top or back is not going to get shellac in it. So what's the issue a future repair person might run into with a shellacked inside? I'm sure there is something I can't think of I guess it just comes down to is it worth the worry?

A coat of shellac should have no effect in regluing a brace in the same spot. There are other issues with that, particularly depending on the glue used, but that's another matter.

The main issue is in crack repairs, where you want to put studs across the crack. Shellac interferes with the glue bond for the same reason it's effective in slowing immediate moisture gain or loss: the glue has to wet the wood surface to stick, and with shellac on there it can't be as effective. Those of us who's arms are too large to get in through the hole to reach the spot and sand back the shellac appreciate the problem more. Cracking on flat top instruments is mostly a long-term issue, having to do with the inherent stability and crack resistance of the wood used, and 'shrinkage hysteresis' from repeated moisture cycling. The short term benefit of a shellac coating won't help that much.

Its odd that arch top makers would tend to use shellac on the inside more than flat top makers, if true. In my experience building both sorts I've found humidity change to be much less of a problem on arch top instruments. The arch might move up and down a bit, but that usually doesn't hurt anything so long as you can adjust the bridge height to accommodate.

Sacconi advocated a coating of water glass on the inside of the plates as part of the 'secret' of Stradivari. This adds a lot of stiffness, particularly across the grain on the top, and may stabilize the wood a bit as well. There are problems with it. One is that the usual sodium silicate is quite hygroscopic and acidic, iirc, and can cause the wood to break down over time. Potassium silicate is apparently more stable. The other issue is that there's not much evidence that Strad used it. Other sorts of chemistry have been proposed for whatever coating Sacconi may have seen that would work better and be more stable, but, again, from what I understand this was not common if it was even used at all.

Sealing the inside is recommended in the most popular book for the archtop guitar. Bob Benedetto copyrighted his book in 1994. It seems many of the builders of archtop guitars like myself took Bob's advice and sealed the inside of their guitars and other builds. It may be that Benedetto has had some influence on other builders, as well. I don't know how many do or don't seal the inside but in this thread at least, there are a few of us archtop guys that are guilty of this deplorable act!

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I shellac the entire "boat" on some of my more expensive guitars with fine sanded interiors. It makes it look good, and I think it will inhibit rapid drying or wetting.
I've never heard of an issue on any of my guitars, I doubt any of them are having braces falling off that will bother a repair man.
Since I have to deal with many issues of bad repairs by others and poor construction by Gibson, Martin, Harmony, and all others, I say to repairmen everywhere: "deal with it, I do".

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I wonder what Bob Benedetto does....

Shellac will only slow the hygroscopic nature of wood and if you Google this you will find multiple references to this in lots of different writings. It's pretty boring reading but it's out there.

Regarding the distinction between rapid RH changes and prolonged RH changes they both can damage a wooden instrument.

As Al mentioned cleating is problematic as well when the maker has used shellac on the inside of the box. We have to remove the shellac to install a proper cleat and that's a pain especially when in the tail block region out of reach.....

The guitar pictured in the bridge regluing thread has a crack in the tail block region that was repaired in Massachusetts by another Luthier. He couldn't reach the crack on the inside to cleat it so he didn't and the crack came open again. I reglued that crack contaminated as it was.... and cleated it with an extension tool that I came up with and named but the name is not fit for a public forum. Our students have seen the tool and may recall it's name....

Al also mentions something that I have not considered and that is a change in voicing because of shellac on the inside. That may not be a bad thing either, just different.

Anyway no biggie, shellac away, we will just do more billable hours removing it when and if your stuff cracks.