Inside the ukulele?

You will find that some of us do and others don’t. The arguments for are; it looks cleaner, it is easier to blow out dust and it slows down the effects of humidity changes. The arguments against are; it makes future interior repairs difficult and it is an additional layer of finish that can adversely effect the sound.
Brad
 
I don't see the point
 
I don't see the point

Well actually there are several potential points: 1) It might look better, 2) it might increase volume and sustain, and 3) it might stabilize expansion and contraction of the sides and back. Plus it is relatively easy to do so why not? Well first of all you have to do it with back on because you wouldn't want to apply finish along the sides of the back where it will be glued to the edges of the sides and the linings because glue doesn't adhere as well to finished wood as it does to bare wood. Just spray once the back is on and then close the box with the top. Sometimes I do it and other times not. Why? Because I'm not sure 1, 2, and 3 are true and it is an extra step so I don't always bother.
 
Sometimes I do it and other times not. Why? Because I'm not sure 1, 2, and 3 are true and it is an extra step so I don't always bother.
1 part science, 1 part uncertainty and 1 part pragmatism. Luthiery is a wonderful mix.
Miguel
 
I do it as sealing the inside slows down the absorption and release of humidity, which minimises cracks.
 
In my experience with instruments that have a sealer or some kind of finish on the inside, a few times in my shop I have had a guitar come in with a finish, shellac that was brushed on, and after some years have past, the finish collected a lot of dust and dirt that turned to an awful mess. I couldn't blow or wipe the dust out. It is for that reason I would not apply a finish to the inside. Also, some of the bracing on the inside was loose, which also had finish applied. What could have been a simple repair, turned into a costly one.
 
From what I've read about furniture making, the idea of finishing the inside of pieces (as well as the backs of pieces that would be against a wall) is a fairly new idea. Say within the last few decades. I remember reading an article by a woodworking journalist who said that he had looked inside about a zillion old pieces of furniture to see how they were constructed, but had virtually never seen an old piece with original finish on the inside. This has been true of every old piece of furniture I've personally looked at, although I haven't looked at enough to be considered any kind of expert. It's now conventional wisdom that both sides of every board must be covered with finish to prevent warping. But if that were really true, there would be no old pieces of furniture that still had flat boards. The reality is that if the lumber was properly dried when the piece was made, it was properly constructed, and the piece was used or stored under decent conditions (e.g. not exposed to water and not subjected to extreme changes in humidity like moving from the Amazon to the desert), the piece can stay stable for hundreds of years.

Of course instrument making is a very specialized form of woodworking, and uses some very thin pieces of wood. But I believe a lot of instruments over the centuries have been made without finish on the inside, and a lot of them have held up well if used and stored under decent conditions. Knowing the conventional wisdom regarding finishes in furniture making is just wrong, I would have no problem skipping finishing the inside of an instrument.
 
I invite people to get a piece of spruce and finish one side only (be it a quick wipe of shellac or spraying nitro etc) ....and see how the wood reacts.

In short, apart from slowing down the intake and release of moisture, finishing the end grain and both sides of a piece of wood (be it a table top or instrument top) is good woodworking practice to equalise the moisture absorption/evaporation rate.

Also, it is more logical to attempt as best we can to minimise cracks occurring then holding to the notion that we shouldn't do all we can as it is a little easier to repair if it does happen.

PS it is easier to repair a crack without a finish on the inside but its only 20 seconds of rubbing with alcohol to take of a little shellac on the inside. As for it collecting more dust...I don't know what to say to that apart from my advice to play the thing and stop looking inside it.
 
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I invite people to get a piece of spruce and finish one side only (be it a quick wipe of shellac or spraying nitro etc) ....and see how the wood reacts.

In short, apart from slowing down the intake and release of moisture, finishing the end grain and both sides of a piece of wood (be it a table top or instrument top) is good woodworking practice to equalise the moisture absorption/evaporation rate.

Also, it is more logical to attempt as best we can to minimise cracks occurring then holding to the notion that we shouldn't do all we can as it is a little easier to repair if it does happen.

PS it is easier to repair a crack without a finish on the inside but its only 20 seconds of rubbing with alcohol to take of a little shellac on the inside. As for it collecting more dust...I don't know what to say to that apart from my advice to play the thing and stop looking inside it.

In my response above, I talked about warping, but the same is true of cracking. There are plenty of, say, drawer fronts, that are as wide as a uke top but have never cracked over hundreds of years despite having finish only on one side. Same with wide panels in frame-and-panel construction, which can be as wide as a guitar top. They only crack if they were glued all the way around, regardless of having finish on one side or two.

It should be noted that not all finishes have the same resistance to moisture transmission, and a light coat of shellac offers very little resistance. So if you're putting multiple coats of some other finish on the outside (or even multiple coats of shellac on the outside) and one coat of shellac on the inside, the situation isn't all that much different than having nothing on the inside. The end grain of top and back panels will be sealed either with finish or glue (from bindings) regardless of whether the inside is finished. The end grain of the sides will be sealed with glue, regardless.

I think that avoiding cracks is more about accounting for wood movement, as well as avoiding extremely low humidity. I haven't actually tried measuring if the bodies of hollow body instruments made from solid wood change slightly in width from season to season, but I would be willing to bet that they do. Sides are springy before the instrument is glued together, implying that they can move slightly once glued up. Design of bracing and the glue used to glue the bracing is likely to also make a difference in propensity to crack. But the wood will move as much as it's going to move as the instrument adjusts to a change in humidity, regardless of the type or amount of finish.

I should emphasize that I really think there's nothing wrong with finishing the inside. It's just a matter of personal preference.
 
I'm a bit of a fence sitter on this one, but I think Beau makes valid points and I do finish the insides (not the top of course), because I figure it will do no harm and could possible do some good. On the flip side, Uke-alot makes a valid point:

The end grain of top and back panels will be sealed either with finish or glue (from bindings) regardless of whether the inside is finished. The end grain of the sides will be sealed with glue, regardless.

As anybody who knows anything about wood knows that the majority of moisture absorbed of expelled is done through the end grain. As a matter of practice, I always seal my top and back endgrain with at least three coats of dilute shellac (1lb cut). Then the endgrain is further sealed with binding and glue. Sealing the exposed longitudinal grain too just seems like a no-brainer to me. My big question has always been; why don't the Big Boys like Martin, Gibson, Gretch, etc. do it? Simple economics? Or do they know something we don't.
 
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