Why not finish the INSIDE of a guitar?

[hamburg325]

This would seem to make it impervious to moisture issues.

[Tim McKnight]

Some of us do use a sealer on the inside. Its not impervious to moisture but it does retard moisture exchange.

[jpbat]

It is common practice in the violin/cello making to use some sort of sealer inside the body.
I always wondered why it is not usual in the guitar world.

[PWoolson]

Finishing the inside can make things a LOT harder on a repair tech should something need to be done down the road.
As Tim said, unless you completely paint the inside (which would add a lot of weight and pretty much kill the sound) you won't make it impervious to moisture.
I did a couple with shellac in my early days and found it to be a waste of time. Mostly because it was almost impossible to get the inside of the top unless I did that before I attached it to the body. Then that joint was unsealed.
And I really didn't find any benefit to sealing it.

[gitnoob]

I recall one luthier here mentioned that he felt that a glassy-smooth interior improved the sound. That sort of makes sense to me. If you want to make a sound baffle, you make something with a lot of surface area. Smoothing the interior with a finish should reduce the surface area -- less diffusion.

[WaddyT]

I don't know about sound, but I do know that Somogyi and some of his students seal the inside of their guitars with shellac. I saw a couple of his graduated students guitars at Dream Guitars, and they were sealed inside. Just an interesting thought here, it probably hides tiny little bits of glue squeeze out. Maybe a plus, from that standpoint. I was told that people who order expensive guitars spend a lot of time with a mirror inside, looking for glue! Then when they find a spot, they call and tell how great the guitar sounds, but complain about the glue!

[mmasters]

It's a different sound, not necessarily a better one though else it would be done a lot more. :)

[mmasters]

I was told that people who order expensive guitars spend a lot of time with a mirror inside, looking for glue! Then when they find a spot, they call and tell how great the guitar sounds, but complain about the glue!

LOL! Where'd that come from?

[Taylorplayer]

>>> I was told that people who order expensive guitars spend a lot of time with a mirror inside, looking for glue! Then when they find a spot, they call and tell how great the guitar sounds, but complain about the glue!

At that point, I would do my best imitation of "You have reached a non working number, please check the number and try again." I mean, c'mon... I would never insult a luthier in that manner.

[garywj]

A couple of my guitars are coated inside.

[sixstringjack]

I have raised this same question several times. As an experienced woodworker i can say that no competent woodworker would finish only one side of a board. To do so would mean the unfinished side would absorb more moisture than the finished side, expand and cause cupping. Lay an unfinished board on your lawn and you will see this cupping in a few hours as the down side absorbs moisture. If the board was constrained, like guitar parts, this unequal expansion results in cracks. Putting a wash coat of shellac on the raw wood doesn't actually seal it, but rather equalizes the moisture absorbtion. Its also easy to do before assembly. I suspect many cracks could be eliminated if the inside were finished.

[Kent Chasson]

I have raised this same question several times. As an experienced woodworker i can say that no competent woodworker would finish only one side of a board. To do so would mean the unfinished side would absorb more moisture than the finished side, expand and cause cupping. Lay an unfinished board on your lawn and you will see this cupping in a few hours as the down side absorbs moisture. If the board was constrained, like guitar parts, this unequal expansion results in cracks. Putting a wash coat of shellac on the raw wood doesn't actually seal it, but rather equalizes the moisture absorbtion. Its also easy to do before assembly. I suspect many cracks could be eliminated if the inside were finished.

Guitar tops are thinner than most anything used in furniture and structurally quite different. That's why you can get away with treating the two surfaces differently.

As for sealing, as Tim said, it won't keep water vapor out permanently but it will drastically slow the transmission. In fact, there was a chart in this months Fine Woodworking (taken from Hoadley, I believe) that showed shellac as one of the better finish materials for slowing moisture transfer. And that's the goal for me and that's why I do it on tops (and sometimes on the entire box). Guitar plates are better able to cope with dimensional changes slowly and are more likely to split with sudden changes. Just a bit of extra insurance but most guitars do just fine without it.

As for repairs, if you put a full thickness finish on then yes, repairs could be more of a pain. But most people just use a wash coat that sands off qick and easy.

[Placida]

This would seem to make it impervious to moisture issues.

Why not just cover the soundhole when you're not playing it?

Wait a minute ..... soundhole covers ..... hole stoppers ...... what an idea! :cool:

[petelor]

I would think the less finish on a guitar the better. I mean why cover an area with sound dampening finish if it doesn't serve to protect anything..?

[epaul]

Alvarez-Yari finishes the inside with a sealer coat of something or other. Or at least, they used to. I have a CY-40 nylon string, and they regarded the sealing of the inside as one of the many good reasons to select an Alvarez-Yari guitar.

The first twenty years of that guitar's life, I paid no attention whatsoever to humidity, (or much of anything else, for that matter). I left the guitar out of its case for weeks on end. I never measured humidity back then, but I expect the house was about the same then as now, which would mean mid-winter humidity levels in the mid-30s, with some low 30s tossed in once in awhile for good measure.


I don't know if using a sealer on the inside helped or not. But, the guitar is in great shape. It looks about as good as it did the day I got it. I don't know what kind of finish Alvarez-Yari used in the early 80s.


.

[vintageparlors]

If I have either the top or back off, I've been known to spray a light coat of shellac inside the body...especially a very dry one.

[SteveS]

Funny you should ask. I asked the same question on this thread (http://www.acousticguitarforum.com/forums/showthread.php?t=13832)posted on 12/13/2002.

[GuitAbuse]

My eastman Ac-415 Jumbo had the inside sealed. Not glossy smooth just a rough coat. It did little to slow the effects of humidity on this guitar. In fact this guitar swelled more than any others I owned back when I didn't have my humidity under control. But I think alot of that had to do with it's light build and large size compared to my Dreads.

[hamburg325]

I own a pair of loudspeakers (Totem Mani 2s) whose inner wood surfaces are finished in the same manner as the outer wood, to equalize acoustical properties. Seems to make sense.

Is the practice of NOT finishing the inside simply a function of stubborn tradition? (e.g. "If Martin didn't do it in 1939, it ain't worth doin.")

Wouldn't a finished inside create a more stable instrument, owing to better resistance to moisture?

[GHS]

I have a couple of old Yamahas from the 70's and the interiors are sprayed with some type of shellac or something. The necks are shot but both bodies are fine. Any one know why,( besides the cost ), this was done and why it stopped? My FG730s does not have it.

[mark g]

I would think the less finish on a guitar the better. I mean why cover an area with sound dampening finish if it doesn't serve to protect anything..?

+1 I agree!

[mmapags]

As others have said (including a couple of luthiers) it is just a thin washcoat of shellac. No discerable thickness and no measurable impact on tone. Not even close to comparable to 12 coats of lacquer. Or even 1 coat of lacquer. Just a little extra protection and, to me, it makes the inside look more complete.

[Tim McKnight]

There is another reason that I apply a wash coat of sealer, that no one has mentioned yet... the wash coat of sealer DOES change the stiffness of the plate while not adding any measurable mass to the plate :cool:

[jpbat]

When I was an apprentice in violin making at Mirecourt at the beginning of the 70's, we were applying a thin coat of gelatine simply mixed in water iside the body. The purpose was slowing the humidity exchange. Not something that could possibly harm the sound. Also the inside of the body had to be at the same level of sanding as the outside, all the angles rounded, etc...