Finishing soundboard

[printer2]

Originally Posted by Earl49

^^ This, and it bears emphasis. Even a thick welded steel hull needs a bilge pump. The surface finish protects from dirt and grime, but is not impervious to moisture transfer. So there is little point to finishing the inside of a guitar. The inside finish will pass moisture too.

As usual, I am quite impressed by the well reasoned background thoughts that Alan brings to his answers.

I worked for a military supplier and we needed to seal a large (relatively) opening and when plastics were brought was told that the only real barrier to moisture is metal.

[dneal]

Originally Posted by gr81dorn

I want to try and make this obvious statement without sounding condescending: There are lots of types of wood and lots of types or products made from wood and depending on what you're using and what you're making, the processes are often going to be different. A guitar top and a tabletop share almost nothing in common other than they can be made of wood - the species, the drying standards, thickness, joining methods, etc....totally different things so you should expect that they are treated differently in the finishing steps, as well. I understand the OP's curiosity and question, so completely reasonable to inquire.

I'll just say that in my opinion, finishing the underside would provide almost no advantages to the instrument and a cost/benefit analysis would 99.9% benefit leaving it raw.

I see your point, but fundamentals of woodworking apply. There is much in common with other wooden products, and you can find the same principles applied. Spruce bracing is not unique to a guitar. Dovetails are not unique to guitars.

Certainly there are some peculiarities depending on the specific application and desired outcome, but to say they have nothing in common is a bit of hyperbole, IMHO.
Certainly

[redir]

A good while back I went on a studying spree of finishes when this topic came up on another forum and Alan Caruth I believe linked to some suggested reading.

Anyway at this point the only thing I think a finish is good for is making it easier to wipe off the finger prints and to some extent helping protect against minor dings. If anything the finish does is make the guitar 'worse' so that's why I like the thinnest finish possible, shellac.

BTW I think the Selmer/Maccaferri guitars finished the inside of the body but not the top. The thinking there was that no one will ever be able to get their hands inside the petite bouche anyway so you may as well try and protect it.

[printer2]

Originally Posted by dneal

I see your point, but fundamentals of woodworking apply. There is much in common with other wooden products, and you can find the same principles applied. Spruce bracing is not unique to a guitar. Dovetails are not unique to guitars.

Certainly there are some peculiarities depending on the specific application and desired outcome, but to say they have nothing in common is a bit of hyperbole, IMHO.
Certainly

The joints may be the same but guitars are built 'wrong' in terms of reducing the effect of the movement of wood and the reduction of cracks. The only reason they are built wrong is to make them good acoustical transducers of sound. A table or chest of drawers are not required to sound good, just to look pretty and survive. An airplane made of wood has more in common with a guitar than a piece of furniture.

[M Hayden]

John Bogdanovich’s book recommends finishing the inside, and it never quite made sense why.

[dneal]

Originally Posted by printer2

The joints may be the same but guitars are built 'wrong' in terms of reducing the effect of the movement of wood and the reduction of cracks. The only reason they are built wrong is to make them good acoustical transducers of sound. A table or chest of drawers are not required to sound good, just to look pretty and survive. An airplane made of wood has more in common with a guitar than a piece of furniture.

I'm not sure if you're trying to contradict my post, but you're illustrating my point. You're listing examples of peculariaties for desired outcome and specific application. Guitars aren't built "wrong". They're built to achieve certain characteristics. My understanding is that luthiers try to prevent cracks, just like any other good woodworker. Controlling for humidity before and after is not ignored by anyone who knows what they're doing. It doesn't matter if it's a lumber yard drying 2x4's before selling them, a homebuilder letting wood flooring acclimatize before installing, or a luthier building at a certain humidity and the player maintaining a certain range of it.

[arie]

i believe that there is a certain amount of surface hardening that occurs with a thin wash coat and a degree of barrier protection. there may also be a small amount of reflectance as well.

classical makers H&A used to shellac and do a light fp on the inside. maybe it was part of their unique tone.

[SouthpawJeff]

Originally Posted by dneal

Good post, but you're mixing two ideas in this part. "Movement" (expansion and contraction) is different from "cupping". A flatsawn board is more prone to cupping than a quartersawn board because one side of the board shrinks at a faster rate than the other (radial shrinkage). Ripping a wide board and flipping on half of it mitigates the effect, because it's a smaller part of the radius, and half of the radius is reversed.

Expansion and contraction, which can result in cracks or split joints happens when you don't account for it. Using a pinned breadboard end on a table is an example of how to mitigate the problem.

Exactly, my point being that cutting and flipping does not affect wood movement from expansion/contraction, it merely reduces the visible affects resulting from the cupping that may result from that movement. The board still moves exactly the same amount as it would had it not been cut. Cupping and expansion/contraction are both effects of moisture change. With no environmental change there’s no movement and/or cupping.... so we’re agreeing here😉

In guitars the soundboard braces are acting similarly to breadboard ends in that they are running long grain across the grain of the sound board. Except they’re worse as they’re glued across the whole width....Generally a no, no, in Woodworking. The reason they work is that the soundboards are quartersawn, narrow, and generally speaking excessive humidity fluctuations are avoided. The finish has pretty minimal affect one way or other. In fact one could very well build a top with a single piece of flat sawn wood. As long as the humidity level never changed it would be fine finished or not. However even if you coated both sides of the top with a thick layer of polyester finish, if you then took it to a much different humidity level, (ie. build in the desert and move to the ocean), that finish will not prevent the top from self destructing.

Lastly regarding finish, different finishes have varying levels of hardness. They all protect the wood though and make the surface significantly tougher than raw wood. Even a light lacquer or shellac finish adds a lot of protection vs raw. That is the main job of the finish on an instrument.
Jeff

[gr81dorn]

Originally Posted by dneal

I see your point, but fundamentals of woodworking apply. There is much in common with other wooden products, and you can find the same principles applied. Spruce bracing is not unique to a guitar. Dovetails are not unique to guitars.

Certainly there are some peculiarities depending on the specific application and desired outcome, but to say they have nothing in common is a bit of hyperbole, IMHO.
Certainly

I am talking very specifically, in the context of this conversation, about creating a guitar soundboard (aka guitar top) and a table top. My point is well made if you keep it in the context and not talk in generalities as you've done. Obviously there is plenty in common, but the specifics of creating one to the other are extremely different and very few meaningful similarities.

[gr81dorn]

Originally Posted by dneal

Controlling for humidity before and after is not ignored by anyone who knows what they're doing. It doesn't matter if it's a lumber yard drying 2x4's before selling them, a homebuilder letting wood flooring acclimatize before installing, or a luthier building at a certain humidity and the player maintaining a certain range of it.

That's only true in broad stroke and misses the point fairly widely - those things all have completely different standards and actually aren't all that similar. Drying framing lumber has a completely different specification than drying lumber to make finish products like flooring, mouldings or instruments or many other things.

Ultimately, the point remains that you can't really consider one product's standards as acceptable or sensible to another's. A 2x4 is made of very specific low grade wood, dried very specifically, machined very specifically. Flooring is made of different species, dried completely differently and machined completely differently...same true for lots of other products.

To my point, these other scenarios being offered have little in common with one another other than being made of wood, so you need to consider the entirety of the product to understand why the standards and practices vary greatly.

[Herb Hunter]

Originally Posted by printer2

The joints may be the same but guitars are built 'wrong' in terms of reducing the effect of the movement of wood and the reduction of cracks. The only reason they are built wrong is to make them good acoustical transducers of sound. A table or chest of drawers are not required to sound good, just to look pretty and survive. An airplane made of wood has more in common with a guitar than a piece of furniture.

I don’t think the term, transducers, applies to acoustic guitars, they are passive radiators. Of course, if they are equipped with pickups, then they are radiators equipped with a transducer or two.

[dneal]

Originally Posted by gr81dorn

I am talking very specifically, in the context of this conversation, about creating a guitar soundboard (aka guitar top) and a table top. My point is well made if you keep it in the context and not talk in generalities as you've done. Obviously there is plenty in common, but the specifics of creating one to the other are extremely different and very few meaningful similarities.

Sorry, but I disagree. I think the commonalities are important, to recall why we finish wood in the first place (and that's the context of the thread). The way you go about finishing a guitar and a piece of furniture might differ in material and method, but the reason is the same.

Originally Posted by gr81dorn

That's only true in broad stroke and misses the point fairly widely - those things all have completely different standards and actually aren't all that similar. Drying framing lumber has a completely different specification than drying lumber to make finish products like flooring, mouldings or instruments or many other things.

Ultimately, the point remains that you can't really consider one product's standards as acceptable or sensible to another's. A 2x4 is made of very specific low grade wood, dried very specifically, machined very specifically. Flooring is made of different species, dried completely differently and machined completely differently...same true for lots of other products.

To my point, these other scenarios being offered have little in common with one another other than being made of wood, so you need to consider the entirety of the product to understand why the standards and practices vary greatly.

I said fundamentals for a reason. Yes, that is a "broad stroke" and was intended to be. The only point being missed is by you. A mortise is a mortise is a mortise. It doesn't matter if you're timber framing, assembling a cabinet door or attaching a neck to a guitar. Of course they're different applications, using different types and dimensions of lumber; but it's still a fundamental and common to all. Controlling for humidity is a fundamental of woodworking.

[redir]

Originally Posted by arie

i believe that there is a certain amount of surface hardening that occurs with a thin wash coat and a degree of barrier protection. there may also be a small amount of reflectance as well.

classical makers H&A used to shellac and do a light fp on the inside. maybe it was part of their unique tone.

Best answer imho. It may have nothing to do with moisture after all.

[Alan Carruth]

Herb Hunter wrote:
"I don’t think the term, transducers, applies to acoustic guitars, they are passive radiators."

Actually, IMO, guitars are precisely transducers: devices for changing one sort of energy into another. In this case they change the somewhat regular application of force by your right hand into sound. With a bit of an assist from your left hand it can even be musical! Note that guitars are not 'amplifiers': they don't use an external power source to reinforce a small signal and turn it into a louder one. All of the power comes in from your hands, and most of it is 'lost' in the process of being turned into sound.

[Earl49]

Originally Posted by arie

......there may also be a small amount of reflectance as well.....

I see this supposition all the time. I've spent a lot of time looking at absorption data for a wide variety of materials (34 years full time in the acoustics business) and near as I can tell, the only difference that might occur in absorption / reflectivity of sealed versus raw wood occurs at 30 kHz and above - a far higher frequency than a normal human ear has ever heard. Our range of hearing is typically 20 Hz - 20 kHz. Even a dog or cat might not hear frequencies that high, except in their youth. It is quite the technical challenge to even measure absorption coefficients at those frequencies.

Off topic, but I believe similar things about the arched or bowl backs of guitars (GS Mini, certain Guilds, Ovations, etc). Many people readily accept that the arch makes a big difference in reflectivity and therefore tone. It might make some difference in cancellation / reinforcement of certain standing waves inside the "box", because of non-parallel surfaces. But I would expect the difference to be subtle - perhaps even imperceptible - in almost every case. The arched construction makes for stiffer back, which would have an effect on overall reflectivity, however. JMO....