Finishing The Inside

[tracy]

Is there any reason not to put a “Finish” on the Inside of a mandolin?

Thanks!!

[banjo1]

A lot of custom guitar builders do this and they seem love the fact that the high humidty in some parts of the US and world do not effect the instruments at all.
Without the humidity change in any acoustic instrument, that sound the same every time.
Proper humidity helps any acoustic instrument.

The dry woodie sound a lot of us love in a mandolin, does not mean the mando is dry(lack of inside moisture)but it is the closest thing that we can describe the sound with.To me a better way to describe this sound is old(played in, vintage), but then you would have to explain that one every time you used it. http://www.mandolincafe.net/iB_html/non-cgi/emoticons/biggrin.gif

[Michael Lewis]

One consideration regarding finish on the inside of an instrument is for future repair work. If the surfaces are covered with finish what would you glue to if you needed to repair a crack? You could only glue the crack but not be able to put in a cleat or patch to reinforce it.

[Luthier]

I like to put a very thin shellac washcoat on the inside to just get a little bit of a "moisture barrier". Each builder has their own opinion on this and each can attest to the pros and cons.
The "big boys" do not have the time to do anything to the innards but I, for one, seem to think it makes good sense to do. Most moisture intake occurs, however, from the endgrain of the wood so the facegrain is not as drastically affected by the moisture.
Don

[Stephanie Reiser]

Not to highjack the thread, but...what causes stain to seep through the back so that it makes an unsightly appearance on the inside, if one were to peer through the F-hole? I have had this occur on a couple of instruments. Am I carving too thinly, or slopping the stain on with too heavy of a hand? (I use alchohol-based stain applied by hand.)

[Dale Ludewig]

Stephanie, the stain comes through the end grain of the curly or otherwise figured wood. If you were able to, I'm sure you'd find that the same places that are the darkest on the outside, especially when wiping on a color, are the same places you have the bleed through on the inside. You can partially eliminate the effect by wiping "not so wet", but some color may get through anyway. I don't personally consider it necessarily unsightly. Besides, people should have better things to do than peering around inside there anyway. http://www.mandolincafe.net/iB_html/non-cgi/emoticons/biggrin.gif

[sunburst]

I've had stain bleed through the back with the softer maples, especially quilted maple. As Dale says, extreem figure contributes because the grain can actually run all the way through the back.
I seldom, if ever, have stain bleed through hard maple, and that's what I usually use (though, of the most recent four, two are big leaf, one is red, and one is sugar maple.)

As for finishing the inside, I don't do it.

I've posted this information before, but I can't remember the source.
Someone did a study of wood finishes and their relative resistance to moisture movement. With one exception, all finishes slow the movement of moisture but do not seal the wood completely. That means it's impossible to finish the instrument so that humidity doesn't affect it at all.
(The one finish that did seal the wood, BTW, was gold leaf.)

[BlueMountain]

Last year I worked on a handcarved tenor guitar from the 30s. The inside seemed to have been painted by the maker with perhaps a weak solution of hide glue. Seventy years later it was covered with dust? mold? on the inside, but was dry. It cleaned up very nicely inside with a damp rag, but remained tacky for a day. Certainly in a high humidity situation, one might expect slightly reconstituted hide glue to be a nice medium for growing various organisms. Shellac seems like a better bet. Merely slowing down humidity changes to the wood for an hour or two can be enough to let one play for a few hours in a too-high humidity situation.

[Jim Rowland]

Egg white on the inside,gentlemen. That is the old way. I don't use anything.
Jim

[PaulD]

A lot of custom guitar builders do this and they seem love the fact that the high humidty in some parts of the US and world do not effect the instruments at all.
I second what John says... I'm unaware of any finish that completely prevents the exchange of moisture. Epoxy is probably as close as you're going to get. If you can flood the wood with thinned epoxy so that it soaks in (as in boat building) you'll create a pretty humidity-proof instrument... but then you might as well make the instrument out of fiberglass, carbon fiber, or plastic.

The one finish that did seal the wood, BTW, was gold leaf.
Finishing the interior with gold leaf would be pretty cool, though.

Paul Doubek

[Michael Lewis]

The answer is that there are several aspects to consider and you must decide which are the most important to you and your instrument. There is no single "best answer" that will satisfy all situations. If there will be no repairs in the future of the instrument then it would be prudent to seal both the inside and outside with shellac, but you just never know if the instrument will be damaged and certainly don't plan for it to happen. There are many instruments that have survived many many years without any damage, but there are also a great number of instruments that have not fared so well.

[Paul Hostetter]

The insides of Selmer guitars, following the lead of Maccaferri's history with Luigi Mozzani, were sprayed with a thin coat of nitro. Allegedly the outsides were French polished, but this photo has to make you wonder. The ventilation for the spray booth here is an airshaft outside an opening in the wall behind - period!

http://www.lutherie.net//selmer.spray.1932.jpg

I don't think it did much of anything. It doesn't really protect the insides from moisture, as if they needed that anyway. It doesn't hurt, and makes it look perhaps a bit nicer. It might have a very subtle effect on the way sound bounces around inside, but it would be subtle indeed.

[Dan'l Terry]

In my experience, it is wise to put a finish on the inside (although there are consequences either way). It's been done on instruments for centuries but was largely abandoned in mass produced factory made instruments for cost and time savings.

An unfinished inside absorbs and loses moisture more readily than the outside, so as humidity changes the inside surface of the wood expands and contracts more than the outside. Over time this results in cracks, thousands of which I've repaired over the years. Others mentioned that finishing the inside prevents later repairs. Nah. A bit of sanding whereever a cleat or repair needs to be done is all that any repair person needs to do to make a glue bond again.
However finishing the inside increases the instrument's sonic reflectivity and tends to increase the higher frequencies more so the instrument will tend to be brighter. For some that's good. For others, bad. It's subjective.

For a less dramatic way of reducing the humidity transfer internally one can coat the inside of the back with plain old hide glue. In classical art this was done to the backs of paintings to reduce the humidity getting in and to even the tension on both sides of the canvas and it works the same way with thin woods. This effects sonic reflectivity less than a finish, yet reduces absorbtion and loss of humidity, thus reducing the dynamic that in time causes expansion cracks.