Finishing soundboard
 

I've often wondered why the insides of soundboards aren't varnished.
Actually Maccaferri did I believe but why no-one else?
Surely it would help with humidity stability issues?
And maybe if the whole of the inside was varnished it would help the sound reflections too...?
The finish on a piano lid makes a big difference to the perceived sound and, of course, both sides of the soundboard are finished.
Any ideas anyone?
Nick

+1. Thanks for askng this question. I have wondered the same thing.

It would be tough getting good glue adhesion of the bracing to soundboard and soundboard to sides if it was finished. Plus it would be a PITA to finish afterwords.

I am assuming because it costs $$$.

You could simply mask off the areas where glue would go before spraying finish.

I just did a search, and this question has come up several times before on this forum and others. Seems some makers spray a light wash coat of shellac inside.

inside job
 

The thinner you can apply the finish, the better your instrument will sound. By doing the inside, you automatically double (or so) the amount of finish on the soundboard.

I have always wondered this too, as finishing the inside of an instrument would (just reiterating your points Nick):

• slow moisture exchange between the wood and the atmosphere
• ‘equalize’ (hopefully) the moisture exchange rate on both the finished and unfinished side of the wood (in an unsupported piece of thin wood, only finishing one side will create a warpage because of this)
• alter the acoustics by providing a smoother rather than a rough finish.

That being said, my guitars without interior finish, and for that matter, most all guitars without interior finish, seem to work pretty well.

Rick

Not sure if this applies . I’m a retired furniture guy but on dining room tables with solid wood tops the bottom of the top may have stain but they have no clear/sealing coat. The reason is wood needs to breath. Temperature and humidity changes will cause the would to crack/split if it is sealed on all sides. Obviously table tops are much thicker but could the same principle apply to guitar soundboards?

If you've never built a guitar, this is s easier said than done

Interesting.

Wood is dead: it does not need to breath.

Wood is hydroscopic, meaning it absorbs and desorbs moisture in response to its environment. As it does so, it changes size - and geometry. There is almost nothing that can be done to prevent it from doing so. At best, one can slow down the absorption and desorption by applying a finish to its surfaces.

Traditional wisdom and practice for things like table tops is to apply the same finish to the visible face as to the not-visible face. By doing so, absorption and desorption are made more uniform between top and bottom faces, reducing the likelihood of the top warping.

Relative humidity is related to temperature, but temperature, per se, isn't very relevant to wood splitting. (At high temperature, glued joints can fail, however.)


As has already been stated, some guitar makers seal the interior surfaces of the guitar. There is no evidence that doing so stabilizes the humidity changes in any practical way. Having finished interior surfaces can impede later repairs, however.

As far as the application of a finish to the interior surfaces making the interior surfaces smoother, that might depend on the maker. Many small-shop luthiers make the interiors very smooth by sanding the interior surfaces of top, back, sides and braces. In such cases, the application of a finish would likely make them less smooth.

Bottom line appears to be that there is no real advantage to finishing the interior surfaces, while adding effort and cost. That is certainly why I don't.

I agree with Charles on this one. I don't see any advantage to finishing the inside. And also agree w/ whoever stated earlier in the post, that Traditional wisdom = a thinner/less finish is better for sound, overall. It would be just adding another layer of material that will in effect dampen the vibration of the top.

My opinion only, of course.

Cheers

Sorry, forgot to address that one.

Many makers go to great effort to reduce the mass, particularly, of the top. Adding a layer of finish to the interior surface of the top adds mass. If finishing the inside of the top accomplishes no identifiable advantage, the added mass should, generally, be avoided.

While guitars are not aircraft, the paint on a 747 airplane weighs about 1200 lb. You get the idea: it all adds up.

Traditional practice on solid wood table tops is not to apply the same sealer/ clear coat to the bottom that they apply on top. You are mistaken. I worked for a company that was introducing their first solid wood top tables and they finished the tops, bottom and sides all of these tops split and cracked. The company hired an experienced production person who quickly diagnosed the problem. I paid particular attention to this over the years and have found no company that seals table top bottoms. I presently own two solid wood dining tables and both have stain for coloring on the bottoms but no sealer/ clear coat. I spent 15 years as a dining furniture specialist working for manufacturers and another 15 years a a retailer and may not have great knowledge of guitar production methods but am very familiar with solid wood table tops.
You may understand hydroscopic, to say nothing can be done to stop the exchange (call it breathing or moisture exchange) may or may not be true but what is true is that solid wood table tops are not sealed on the bottom and if you do so they will split.
My question was if this may apply to soundboards.
Solid wood table tops are generally .75 to 1.25 inches thick compared to a very thin guitar soundboard and this may account for a significant difference in allowing these two breath or exchange moisture without splitting even if soundboards are finished on both sides. Wood for furniture and likely guitars too have to be dried not to exceed a particular moisture content before production. Because of the thickness of table tops the wood must be cut into pieces that are about 3” to 4” in width then after reaching the correct moisture content that are glued together to form the size of the top. This is done because a sheet of solid wood cannot be dried to a consistent moisture percentage in a pice of wood that size and thickness. This again may be the difference with a soundboard and a table top and the ability for a soundboard to not split if both sides are sealed.

It is true that antique pieces were not usually finished on interior/underside surfaces. (Many were still rough-sawn/rough-planed on interior/underside surfaces.) Many cracked, often due to construction methods that didn't take into account wood movement.

I have two large dinning room tables - one cherry, one oak - both solid-wood, handmade by local craftspeople, both finished top and underside - stained and lacquered. Neither has cracked in 20 years. In modern times, it isn't uncommon to find handmade solid-wood furniture that is finished on the top and bottom surfaces of a table top, for the reasons I mentioned.

Clearly, there are examples of solid-wood furniture with different finishing treatments - some with finish on table tops only, and others with finish on both top and bottom surfaces - that have survived without cracks and those that have cracked. If you survey furniture makers and wood technology experts, opinions vary on the best practice.

Perhaps more important than finishing methods are construction methods that allow the wood to move in response to seasonal changes. Guitars are made with the antithesis of that. I can't speak to the construction methods of the furniture of the company where you worked, but it might well be that that was a factor.

Lastly, customers would be unhappy if they looked at the underside of their table top to find that it was a different color than the top and is one of the reasons that manufacturers stain the underside, while saving money by not applying a top-coat to that surface.

My point was that applying a finish on all surfaces of solid wood does not cause wood to crack or split. There are many, many crack-free examples of articles made of wood, some of which are table tops, that are finished on all surfaces.