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1st time builder here........what sort of "finish" is normally applied to the inside of the guitar?

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Steve Leonard
SEleonard

None. The inside does not normally receive any type of finish.

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Remember, there are no stupid questions, only stupid answers!

Paging Dr. Breakstone....

I was surprised at the answers in a FB ukulele making group where this question was asked recently. A high number of people said they wipe shellac after the braces are all on.

Reasoning varied from protecting the inside of the box to humidity protection to looks nice.


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So, here is a way of looking at this issue:

1. Most acoustic guitars (steel string, classical, whatever) do not have finish on the inside of the guitar. That's just a plain fact.

2. Yet, there are some people who do, in fact, put some finish on the inside of the guitar, for a few different reasons that make sense to them. So, it is not a crazy, unheard of thing.

3. The people who do a lot of thinking about this sort of thing can have strong debates and advocate for one way being better than the other.

4. As a new builder, I would try to make my first few guitars as much like the "norm" as I can, and not take on a bunch of extra work or add extra features. Walk before I run, etc. So, I would not want to put finish on the inside of my guitar. I would wait and think more about that after I know more.

I hope that helps.

I put some Thompsons Water Seal on a couple of mine, but then I live in a Gulf coast Soggy Bottom. No joke

Generally no finish on the inside of the box:) I do agree w DC that it would make sense to keep things simple and as close to normal as possible w a first build:)


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Robert Benedetto does wipe on finish on the inside of his boxes, as can be seen on his “Making an arch top guitar” DVD.

Back in my furniture days, it made a difference to finish the underside of a table the same as the top, and it helped keep things flat more often. No need for polishing or anything. But keeping the rate of transfer of water vapor similar on both surfaces of a piece of wood was a good thing.

I have yet to hear of a finish that "seals" the wood from water vapor transfer, though many claim something like that, and lots of folks believe they do. To do that, the finish would need to keep out a water molecule, and I don't see it happening at that level regardless of the finish applied.

So we are only talking about slowing down the transfer, not stopping it. And in this case just balancing the rate of transfer between the outside and inside of a guitar.

Surprisingly, shellac is one of the better finishes at slowing down the transfer of water vapor, as old a finish as it is.

So, in my opinion, a coat or 2 of shellac on the inside of a guitar is a good thing.

Except for the fact that glue doesn't stick to it well so internal repairs down the road will probably be much more difficult.

For that reason alone, I don't finish the inside as much as I'd like to.

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Brian R, Wood Mechanic
N8ZED

I use shellac for the inside and outside, too.
Or shellac for the inside and Nitro for the oustide
I always put shellac on the inside. Makes lots of sense.
And what Ervin does can not be wrong.....

The thickness of the film required to balance moisture transfer for interior and exterior would be significantly more than what is typically seen here on instruments with interior finishes, so it seems to me that in most cases, the purpose is appearance and/or tradition...and it certainly seems to reduce the time spent cleaning up an interior prior to closing the box. Interior finish does dramatically complicate some repairs (e.g., crack repairs/other gluing to interior surfaces), but with scraping and other mechanical film removal, cleanup may be accomplished and the necessary work done with the associated charge against labor.

We shellac specific, very limited areas of the interior where we will be installing wiring stand-offs, preamps, velcro battery pads, or other PSA-faced components such as the Anthem system microphone. After masking with 3M green (the new product number is 233, which I believe is the old 2060 tape), three to four coats of 1 lb shellac raises the grain (first application), then seals (subsequent coats), providing a more reliable adhesive bond for PSA. We do not do this for K&K/JJB/other CA-mounted transducers, as the BSI gel or standard medium CA does just fine with a bare, freshly-scraped or sanded wood surface.

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So far no-one has addressed the issues that might be put in favour or against finish on the inside. I will contribute some:
FOR: looks nice through the soundhole, might protect against humidity change (but evidence suggests that it doesn’t), makes it easier to clean inside (but who ever does that?), might smell better (do you smell guitars, I know I do....), indicates an instrument of distinction (because, hardly anyone does that, right?).
AGAINST: extra effort (I mean, nobody else does that, right?), might interfere with glue action in future repairs, shellac - too difficult, expensive, I don’t have the equipment for that.

We do a wash coat of shellac (2lb cut) on the interior of all our bodies. The thought is to help "slow" the moisture change.... not "stop" it. As mentioned above, it would take too much finish on the inside to try and prevent moisture change. My thought is that slow changes in moisture give the wood a chance to handle it gracefully. Or, in some cases, it might help the guitar not absorb or release as much moisture when put into a different environment for a few hours... like doing a gig outside, or visiting a studio that's unusually dry, etc.

I agree that most guitars that exist today do not have any finish on the inside. But, I think the primary reason manufactured guitars don't is because it's added cost, and their guitars are typically overbuilt to help deal with potential problems.

I did a lot of repair work in the dissipated days of my misspent youth, and I always hated to see finish on the inside. It makes any sort of repair a lot more time consuming, and can't be properly touched up, so however nice it looks at first it won't be so nice down the road.

Rapid moisture change is usually not as much of a problem as it's made out to be, especially if the instrument is made at a lower R.H. than it's expected to live in. I'm in New England, guitar hell, and our big repair season for cracks was not in the early winter, when the heat came on and humidity dropped fast, or in late January when the low outside temperatures gave the lowest indoor R.H.; it was in February and March, when the instruments just couldn't take it any longer.

It was not the new ones either. Wood undergoes 'shrinkage hysteresis' over time. The part of the wood that absorbs moisture from the air, the hemicellulose 'filler' in the lignin 'glue' that holds it all together, breaks down slowly over time. As it does the wood simply can't absorb as much moisture at a given R.H., so it tends to get a little narrower (wood doesn't shrink much in length) with every passing year. Eventually that builds up stress, and, at some point, the wood cracks. There is not much you can do about this, and a bit of shellac (which is the most effective coating in that regard) won't help.

The US Navy uses a lot of small (40'-50') boats, and up until the 60s or so they were made of wood. A lot of effort went into keeping them seaworthy, so they did studies on water proofing to prevent rot. They found that even a 1/4" thick coating of epoxy won't keep moisture out. After that they went over to fiberglass boats.

Thank You Al - great reply and especially the part about the 1/4" epoxy not keeping moisture out. That says it all.

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Ann Arbor Guitars

With all due respect... no one is submerging their acoustic guitars in water for decades. I agree that putting something made of wood under water will change the game in an inconceivable way. But, custom, hand-made guitars are mostly kept at a humidity norm that humans find comfortable. To me, the issue is when they are subjected to extreme variations over brief periods of time. If there is something reasonable I can do to help with those cases, then it's worth the extra 10 minutes of time.

All that said, I agree as well that an extreme change in humidity over a period of several months is something we are unlikely overcome, much like the boat in the water. Wood, is... after all, wood. It will absorb and lose moisture as it will no matter what we do.

Maybe it's as simple as: "an ounce of prevention..."

Since you made a strong argument and, I assume, you have a significant clientele in the NE, what RH do you build at that you feel insulates your instruments from warranty issues?

I have Somogyi's books and I know he finishes the inside of his (or at least he did), but many first rate makers have odd ideas. One of my favorites was from Jimmy D'Aquisto. He figured that if narrowing the opening of a gardening hose caused water to shoot out farther, then an oval soundhole should cause sound to project farther. Truth or a luthiers edition of Night Gallery ??

I'm not a prolific builder of guitars; I do too many other things. These days I try to keep my shop at around 4-%-45% R.H. year 'round, and have had few problems since I got that under control, with one notable exception. That was was a just-completed guitar I put into a gallery show a few years back. I talked with the organizer at some length about humidity when I dropped it off, and left with the admonition that 'if you're comfortable, the guitar should be'. That was in mid-December. The show opened the first week in January, and it had gotten very cold. When I went down for the opening it was obvious that all of the wooden objects were in some distress. When I spoke with the organizer about it she said: "Oh yes, it has been very uncomfortable lately". The R.H. was about 15%, as near as I could tell.

I went right out and tried to find a humidifier, but could not in any of the local stores, so I gave her some money on the spot to get one, which she did the next day. Unfortunately, the following weekend was MLK day, they did not come in to fill the humidifier, and the outside temperature stayed around zero F. Two days later I got an e-mail that the top of my guitar had cracked. When I came in to collect it I pointed out all of the wood items in their permanent collection that were showing distress, including some very fine furniture and fancy turnings. This was the first showing they'd done in a their venue in a new building, which was much tighter and less humid than the old one. I ended up having to replace that top. I'll note there were two other acoustic guitars in that show (one of mine) that were not fresh off the bench, and they held up reasonably well, with no cracking, although the tops sunk, and the grain looked like corduroy.

From what I can tell, a guitar is usually able to handle an R.H. around 15%-20% lower than it was built at. Higher humidity is normally less of a problem, so it seems to be best to build at a level that's a bit lower than 'average' for the area. People tend to be most comfortable at around 50%. also, iirc, wood moves less between 40%-60% than it does at higher or lower humidity, but I may be wrong about that.