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I didn't come across anything using a simple search, but has anyone seen an actual test of finished vs. unfinished internal boxes? I'm curious to see what would happen if you took one of each, each strung up, and each having been built at 45% R.H. and then brought them into a room of 10% RH. Would there be much of a noticeable difference between the two when subjected to a more extreme environment?

Unfortunately, I don't have any guitars to sacrifice.

I did a few rather unscientific tests. Basically monitoring movement of one finished and one unfinished (inside) guitar, and found the finish inside to be of little help.

i have long wondered why acoustic instruments weren't finished on the inside, as doing so surely must minimize water absorption/evaporation....i could also imagine it increasing treble response...

I suspect they would crack as readily as unfinished ones.................a bit slower maybe but will still crack.
Tom

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Newbie question here: how common or advisable is it to hit the back of the sound board with shellac to stabilize and " toughen" it up as is often done with the front face earlier in the construction process?

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Doug

Don't let fear or common sense stop you from trying to build something

I have not used a finish on wood that would stop the expansion and contraction caused by swings in RH. Some may slow it down, but not stop it. A cabinet door is finished all sides and the panel still moves considerably.

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Brian

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

https://www.howardguitarsdelaware.com/

I think there was a discussion about this last year and what I got out of that is that as B Howard mentioned the finish doesn't prevent a moisture increase but it does slow it down. So if you are flying from Portland to Phoenix the finish could help that sudden and drastic change. I've never done it simply because it's just not typically done that way. I think Selmer guitars have a coat of shellac on the inside. I played a classical guitar that had an obvious coat of finish on the inside. IT was a higher end Takimine and it sounded ok. I'm not sure I would ever do it, too risky and in all the years of guitar building it has never been an issue. Keep it in a good case - problem solved.

What JF said plus, it makes the absorption more even on both sides. A very simple experiment - take two normal width pieces of top wood. Put a couple coats of shellac on both sides of one of the pieces and a couple on only one side of the other piece. Now take them into an environment with a different humidity from where this was performed.

Eventually, they'll both flatten out again but watch how the finished one side board curls up while the finished two sides board remains much flatter.

Ultimately this might not be that big a deal but the larger the instrument, the more impact it's going to have. There are millions of guitars that have been made that are only finished on one side that survive just fine. I've started shellacing mine.

Something else to consider - shellac is a very good moisture barrier. Much better than nitro and most other man made finishes. If you're using a wash coat of shellac on the top side before anything else, it may be more important to balance that on the other.

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Oh, and here's some links:

viewtopic.php?p=492291#p492291

Document about the effectiveness of finishes against vapor: http://www.fpl.fs.fed.us/documnts/fplrp/fplrp462.pdf

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

I think the moisture barrier thing is a myth, and that it is really done to hide glue squeeze-out!

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Waddy

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Sound Clips of most of my guitars

On what exactly are you basing these opinions?

p.s., I agree that 3 coats of shellac won't "balance" moisture exchange compared to a face with shellac and 6 coats of nitro but, it will be more balanced than e.g. a face with a shellac seal and 6 coats of nitro compared to a bare face. Luckily, I have data to back up my position.

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Cause I can steam a dent right out of a French polished top, and recover the finish with minor effort.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

So how do you reconcile your anecdote with the data presented in the USFS report?

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Andy is correct on this.
As a furniture builder of 25+ years I've often wondered this myself. I've always followed the rule of finishing things equally on both sides because of the "warp" issue. There have been numerous articles in Fine Woodworking and other wood publications over the years concerning this. Tests have been done where 30 something coats of finish (way more than guitars) have been applied to a piece of wood to see if it could be totally sealed from moisture. Eventually the moisture content changed and the wood expanded/contracted...
Now having said that, I think sealing or finishing the insides of a guitar does change the acoustic properties. (woody to a more trebly tone as mentioned above). I've noticed a difference after finishing (usually 5 coats)even a bookcase or media center because you are closing the pores of the wood to a certain degree. (like removing carpet in a room, the room's sound changes. "More cowbell Gene!"). And since finishes dry to a hard reflective coating as opposed to say rubber roof coating it adds a trebly tone to it.
Now as Todd mentioned, I think that what is probably one "washcoat" of shellac is probably done just to pretty up the inside. Since it is hardly accomplishing anything to the same degree as the 8-10 coats of nitro on the outside.
There are other questions that keep me awake at night as well such as cross grain gluing of woods (bracing) which is a big no-no. But eventually I drift off to sleep knowing that millions of wonderful guitars are alive and well regardless of "rules" broken.

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Bit off topic, but can you expand on your technique Waddy? (Would heve thought steaming out a dent would badly screw up FP)
thx

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Dave
Milton, ON

I have a very clear imprinted memory from a long time ago (circa late 70's) when I was looking for an upgrade from my starter acoustic...

said memory is of playing lots of guitars and coming across Yairi's stuff...noticed the insides of their guitars were finished (pretty heavily, certainly NOT a washcoat...and there is a very clear image in my head of that)..I also remember those guitars as having a very trebley/tinny (almost reverb) sound...more so than Takamines of that era...as a note I ended up finding a D-28 that was one of those 1 in 100 guitars that was a true winner, working my butt off and being lucky in that when I had accrued the funds it was still in stock...

I prefer a very balanced instrument, but given the choice would prefer one with more bass as opposed to more treble...

the anecdotal point to this is that I am in the camp that finishing the inside of an acoustic does affect the tone, and my view is that it is to the negative...perhaps a single wash to the back of the top (considering it is what is doing the 'work') might have little if any impact on tone, but I think that finishing the backs and sides will result in a more reflective surface and have a noticeable effect...

anybody care to make a slew of guitars (to get a large enough statistical sample) and get some real empirical data?

Purely anecdotal but I left the interior on my first few instruments bare and have since been using a wash coat of shellac. From what I can tell it does noticeably slow down movement from sudden RH changes (which is why I do it). Obviously you're not going to stop the exchange of moisture but I do consider it an advantage if, for example, it takes a top a couple of hours (shellac) to show similar movement to a bare top at say 30 min when taking it from a controlled, 50%ish RH environment to a 15-20% environment.

As for tone I can't tell a difference and I doubt anyone else could but, again, I'm only doing a wash coat to slow moisture exchange a bit. If, however, one were to actually build a finish on the interior I have no doubt that you'd be able to hear a difference.

I wouldn't know one way or another if finish inside a guitar will affect sound, but like Alan, I have been building furniture for 20+ years, and guitar building techniques fly in the face of many common woodworking practises. I wouldn't dream of building a table and not finishing the underside the same as the top. What little I know about building guitars does make me scratch my head a lot!

Alex

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It does make a place in the finish, but it's easy to build it back up, in my experience. I have done it on a number of guitars, though none more than a few years old.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Sorry for the late reply, but thanks for your input. I thought there would be more rigorous testing on this - perhaps it's out there and I need to dig a little more. Maybe Mr. Taylor has researched this. I wonder if he personally answers emails..........

Not sure I want to reduce the time it takes the guitar to get to equilibrium with the environment. My guitar goes out of the house at least twice a week and then has to stabilize wherever I'm playing at. It usually takes about 30 to 45 min (this time of year). As a result, the guitar is stable by the time I get everything setup, we finish sound checks, and run through some songs. During that time the tuning drifts but afterwards the guitar stays in tune. Anything that slowed that process down would result in a guitar that would be changing noticeably during the performance when all I have time for is a quick tweak here and there.

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Steve Smith
"Music is what feelings sound like"

I think that is a really good point. The same thing that slows down absorption also slows down stabilization. Nothing worse than an instrument that starts changing half way through an outdoor gig. I do most of my playing in the outdoors in the fall of the year with lots of humidity and temperature changes. My guitars - unfinished on the inside seem to do quite well. The only exception was my fault. Played three weekends straight in the rain, took the guitar home and left it in the case for the week. By the 3rd week it was pretty badly corrugated on the top. Took it home, took the strings off, and hung it in the shop for a few weeks and it was fine. Nothing came loose. The top seam does show now, though, but it is still my player. My second guitar.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

I've read Bogdonovich's excellent book on classical guitar building. I notice that he brushes shellac on the inside of back and sides. Given how cleanly he builds, he's definitely not covering up squeezeout. It seemed like he always does this, so maybe contacting him and asking if he believes it makes a difference tonally would add to your data base (again, he's not putting a finish on, just a single, brushed on coat).

He also does a coat on the inside of the soundboard.

I see your point. However, and I'm not sure why this is exactly but, even though I can tell that my guitars with a wash coat inside are moving with RH changes (again more slowly), I find that I only have to make very minor tweaks to the tuning (if at all) when moving to and from extreme RH. My instruments that are bare inside, without a doubt, go much more out of tune overall under the same circumstances.

I'm not telling anyone that they need to convert. I don't think it makes a huge difference one way or the other and obviously there are a TON of really great instruments out there that are bare inside. But, having done it both ways and living in an area that sees wild RH swings, I do prefer a wash coat.