Finish Inside of Guitar?

[captwingflap]

Anybody know why the inside of a guitar or other instrument is left unfinished? It would seem to me that sound waves would be amplified by a smoother surface. Just wondering

[Bowie]

Would probably zing with high frequencies and sound unpleasant. Just purely a guess though, curious if anyone knows from experience.

[ryanhoots]

I'm guessing it's tradition, cost saving, and I agree, there are probably acoustic concerns. But I'm no luthier, by any means.

[Ned Milburn]

Some, but few, builders finish the inside of the instrument with a light coating of shellac.

The body of the guitar acts like an echo chamber and the surface will help determine the reflective properties. Unfinished wood, especially softwood, can tend to become "fuzzy" over time (guessing that humidity changes bit by bit help fibre ends stand), thus dulling the reflection. Hence, some people's choice of lightly finishing the inside of the body.

Differences in tone will be only slightly audible, if audible at all.

[verbs4us]

Originally Posted by Bowie

Would probably zing with high frequencies and sound unpleasant. Just purely a guess though, curious if anyone knows from experience.

On a trip to Edinburgh, I tried one of Rory Dowling's smaller guitars (Taran guitars) and it had a light finished on the inside. Depsite the instrument being made of maple, it was warm with glowing sustain and good balance across tones. If I had had 2k pounds in me pocket, it would have come home with me. I have heard other say that interior finish retards moisture exchange in the wood.

[fazool]

Originally Posted by verbs4us

...I have heard other say that interior finish retards moisture exchange in the wood.

My (former) 1977 Yamaha FG-512 touted a moisture-barrier coating on the inside.

[Tom West]

Originally Posted by verbs4us

I have heard other say that interior finish retards moisture exchange in the wood.

That is one reason it's done, but I think it slows it down rather then retards, also it gives a more finished look to the interior. A bit of down side is that to do repair work one may have to remove finish to ensure proper glue adhesion. I have only heard of shellac being the finish used, also very lightly. Personally I think it does not impact tone. As always, just my opinion.
Tom

[Rodger Knox]

Originally Posted by Tom West

That is one reason it's done, but I think it slows it down rather then retards, also it gives a more finished look to the interior. A bit of down side is that to do repair work one may have to remove finish to ensure proper glue adhesion. I have only heard of shellac being the finish used, also very lightly. Personally I think it does not impact tone. As always, just my opinion.
Tom

Exactly correct, except for semantics. Slows down and retards mean the same thing, it should read "slows down rather than prevents"

[SteveS]

Builders that finish the inside do so for various reasons. One builder I know does it only for looks. I decided to finish the inside of my builds, but I'm not building right now. I add a coat of shellac for tonal reasons - to add stiffness and make the inside more reflective instead of adsorbing of sound waves. I think it works for me, or I would not do it. It might not work for others, so they might have good reasons not to. I also like that the wood is a bit more protected and it is easier to clean dust bunnies.

[SanzoneGuitars]

Great thread, seems like most makers do finish the inside these days. Even some of the factories have followed suit. I've tried it and did not like the sound, too glassy. Felt like it reduced the woodiness. People say that the thin tops many luthiers are using NEED an internal moisture barrier as they are more susceptible to change. Pre-war Gibson & Martin instruments weren't finished inside and they hold up just fine after 75 years. Proof enough for me.
Bruce Hoadley is a well-known wood technologist who wrote the book Understanding Wood. There is a good chapter on drying wood in which he discusses the amount of moisture that is retarded by various types of finish - turns out the only real effective moisture barrier is parrafin. Great book if you haven't checked it out.

[SteveS]

Originally Posted by SanzoneGuitars

..... I've tried it and did not like the sound, too glassy. Felt like it reduced the woodiness......

Yep - get 10 builders in a room and ask about this and you will likely get 10 different opinions. I believe as you say that on your guitars an inside finish does not work. I do know that on mine it works - not glassy and more woody.

Originally Posted by SanzoneGuitars

..... People say that the thin tops many luthiers are using NEED an internal moisture barrier as they are more susceptible to change...

I build thin - .070 to .085" and did not find that the top needed a finish for that reason.

Originally Posted by SanzoneGuitars

.....Pre-war Gibson & Martin instruments weren't finished inside and they hold up just fine after 75 years. Proof enough for me....

Absolutely.

[fazool]

I think a nice solution would be a satin(y) clear nitrocellulose spray (perhaps even aerosol). It could be applied with a "fogging" technique just to get a light coating for moisture barrier. Nitro is flexible, clear is....well... clear, and a satin finish might soften that hard reflective surface effect a little and not look awkward, especially when fogging and not polishing afterward.

In fact, if I ever do a BTO I might ask for that.

[WaddyT]

Some repair people arbitrarily charge an up charge for guitars finished on the inside if there are interior repairs required. I saw one that quoted an extra $300 if braces or bridge plate had to be re-glued. Nitro would be the worst, as it would require excessive sanding to remove. Might even change the sound of the instrument to the worse.

[SanzoneGuitars]

We actually tung oiled our acoustics at the RV school back in '95! Mine was redwood too, soaked it right up. Finishing the inside from the outside, yikes..

[Tavymusic]

I finish mine (mostly mandolins though so treat with caution!), and feel it improves the sound slightly: maybe a little more sparkle and clarity.

The main reason I do this was that after restoring a bunch of 100+ year old mandolins with unfinished tops in shocking condition, it seemed to me to be a good way to keep the dust bunnies and stray fibers in place. Come to that even a 70's Eko I worked on was pretty horrible inside - loose fibers hanging down all over the place.

Otherwise the top ends up like this: