Prettying up the back!

[Nahil.R]

Just a quick question about you finish the back (on the inside of the soundbox).

I have sanded down to roughly 300 grit and am contemplating a light wash coat of wipe on poly to give the wood some character.

What are your opinions on this as I've seen backs that are coated and others just sanded. Probably a silly question and inconsequential but I just wanted to get a feel for what most of you fine luthiers do.

Thanks,

Nahil.

[redir]

Some do, most don't. You will find all kinds of arguments for why or why not to and each argument can be refuted. I believe Somogyi gives the back a light coat of shellac. I've seen more classical guitars with this feature then steel string.

You would of course only want to do this once the braces are all glued in because you don't want to glue braces on top of finish.

So IMHO if you think it will look better and you want to do it then go for it.

[charles Tauber]

I think redir accurately and succinctly stated it. You can do it or not, as you prefer.

[Nahil.R]

Thanks guys, I opted to just put in a lot of elbow grease and sand upto 600 grit.

Not going to go with a finish inside.

[charles Tauber]

Originally Posted by Nahil.R

Thanks guys, I opted to just put in a lot of elbow grease and sand upto 600 grit.

Veteran guitar maker Sergei de Jonge spoke about interior smoothness. While he is always very entertaining to listen to, his stories of "roughening-up" the interior surfaces of a guitar using rice, sand and other "abrasives" was very interesting.

These are from my notes/transcripts taken at one of his talks on Guitar Day, September 2019. (Guitar Day was cancelled this year, for obvious reasons.)

Inside the guitar are all these shiny smooth surfaces there was something similar going on with the sound waves bouncing off these shiny surfaces because of too brash a tone. We went to the trouble of building a machine that we could mount the guitar on and add a motor with a cam so the guitar could be shaken and we put about 5 tons of budgey [bird] gravel and shake away for about two hours until all of the lacquer had been removed, strung it up and now it sounded really rich and warm but too much if you were playing Bach you couldn’t hear the inner voices very well it was just too warm and rich and too bassey. So then we put something finer in the guitar, then put it back on the machine then strung it up and it sounded great.

In 1984 or 1985, Yamaha had starting making in the Hamamatsu factory, although at that time they were making most of their guitars up to the $1000 in Taiwan, but they made the very high end ones in Hamamatsu in Japan and they were making a grand concert guitars there, some of the cheaper ones were made in Taiwan, these were the really high end ones. One of them was made with a spruce top, made by one guy there, the cedar top ones were made by another guy there, specializing. They were really good guitars. Yamaha was giving them to really top players to play Leo Brower had one, in Canada Liona Boyd had one. Liaona really wanted to play it, Yamaha gave her some really good incentives to play it, but she didn’t really like the sound, anyway, she brought it to me to see what I could do with it. This guitar was also lacquered inside. Not just lacquered inside but actually buffed. This guitar you literally had to wear sunglasses to look inside this guitar. Anyway I didn’t have budgy gravel, but I had lacquer thinner. By the way the finish on the inside of the guitar was actually lacquer, was nitro lacquer, the outside was French polished and very nicely French polished, anyway I put about a litre of lacquer thinner in it, first removed the label, put lighter fluid on the label and got it off, put a litre of lacquer thinner a swished it around and ahh, it was took all the lacquer of the inside but now it was a mess inside because the lacquer thinner had rosewood color all over the braces and insides. Took me a bit of time but I got it all out. I put some sand or something finer, put it through the process [of shaking the abrasive inside the guitar], strung it up and liona loved it and she was playing it for a while.


Larrivee allowed those working with him – while in Toronto – to make their own guitars after-hours in the shop. George Gray, on the first guitar he had made after hours, sanded all of the interior surfaces to 600 and it was very smooth. I told him that sanded that fine, the guitar would sound too brash. George called a few weeks later and said the I was right that it was too brash.

Sergei said that he, "Sands inside braces to 80 with an orbital sander, about the same as 100 grit on finishing sander, about the same as 120 by hand sanding". On classical guitars he goes finer, since, "Steel strings sound somewhat brash already". On classical guitars he goes to 100 or 120 with a random orbital sander on back and sides.


By contrast, I was taught to sand all interior surfaces very smooth - 320 or 600. These days, I sand to 220-ish - depends on make of sandpaper and hand vs. machine. I've not found my instruments to be what I'd describe as "too brash". Regardless, Sergei has a lot of knowledge and experience, far more than I have. Thought you might find his experience interesting.

[Nahil.R]

Originally Posted by charles Tauber

Veteran guitar maker Sergei de Jonge spoke about interior smoothness. While he is always very entertaining to listen to, his stories of "roughening-up" the interior surfaces of a guitar using rice, sand and other "abrasives" was very interesting.

These are from my notes/transcripts taken at one of his talks on Guitar Day, September 2019. (Guitar Day was cancelled this year, for obvious reasons.)



Sergei said that he, "Sands inside braces to 80 with an orbital sander, about the same as 100 grit on finishing sander, about the same as 120 by hand sanding". On classical guitars he goes finer, since, "Steel strings sound somewhat brash already". On classical guitars he goes to 100 or 120 with a random orbital sander on back and sides.


By contrast, I was taught to sand all interior surfaces very smooth - 320 or 600. These days, I sand to 220-ish - depends on make of sandpaper and hand vs. machine. I've not found my instruments to be what I'd describe as "too brash". Regardless, Sergei has a lot of knowledge and experience, far more than I have. Thought you might find his experience interesting.

Wow, thank you so much for taking the time and sharing your experiences and Sergei's insights, I had never thought of the inside being 'too' smooth as an issue, but I guess it must change the way the sound waves reflect off of it.

It amazes me how all the little intricate details play a role in shaping what a guitar becomes.

Cheers,
Nahil.

[redir]

Yeah that is definitely interesting. I have never seen a Yamaha that was finished on the inside. I do recall seeing some Takamine classical guitars that were and I remember thinking they sounded like good guitars.

Finishing a guitar has an obvious affect on the tone so there is something to be said there. Just don't 'finish' the inside of the guitar. Like I said I think Somogyi does a wash coat of shellac but that's it. Just enough to keep it clean. But maybe he does it for tone too IDK.

Personally I scrape the inside of the back of the guitar because I find it fast, easy and efficient. I have heard arguments for planed vs sanded surfaces too. Something along the lines of, a sanded surface leaves striations behind which are little flex joints sort of like kerfed linings while planing (scraping) the surface cuts the cells of the wood clean and so you have a more open sound... What ever that means

In theory though it does make some sense that a planed top or back could be thinner and stronger then a striated sanded one. Of course if you use an orbital sander you don't have long striations too.

[MThomson]

I can't see how the back roughness drastically affects tone. The wavelength of sound waves is in the cm to m range for audible frequencies. The ranges of roughness we're talking about are 2 orders of magnitude below this and unlikely to have any effect beyond hearing what you expect to hear

[Nahil.R]

Originally Posted by MThomson

I can't see how the back roughness drastically affects tone. The wavelength of sound waves is in the cm to m range for audible frequencies. The ranges of roughness we're talking about are 2 orders of magnitude below this and unlikely to have any effect beyond hearing what you expect to hear

I trust in the experience of those who have taken the time and effort to test different parameters, such as the coarseness of sanding grit (or scraping as Redir mentioned).

If Sergei De Jonge (more than adequately qualified) ascertains that there is a difference in tonal qualities based on the grit of final sanding then I would take his word for it. In the world of luthiery, every little details matters and ultimately affects tone and the overall tonal characteristics of the instrument.

Whether the degree of influence is substantial or not, it's always beneficial to look into the experiences of luthiers whom have honed their craft over many years; and try to work based on their experiences. I firmly believe that is how the craft has advanced itself, and determines which theories work and which do not.

The artform of luthiery is in a perpetual state of improvement and evolution and it is fascinating to follow and be along for the ride.

Nahil.

[CaffeinatedOne]

Originally Posted by MThomson

I can't see how the back roughness drastically affects tone.

My perspective is from doing pro sound for about ten years, where I learned that the oddest details could affect how a room handles sound. I learned early on that an empty room is radically more touchy than a room filled with people. But other things could have bizarre effects. One church had a bad reputation amongst sound engineers for dead spots at unexpected places. I found that room to be very tricky to control during a concert and we tended to control audience placement by cordoning off the rear seats unless we had a full house.

The wall material and design could have huge effects on the behavior of sound. Generally, the less regular the wall design and the softer or more varied the material that made up the wall surface, the more controllable the sound would be.

I found in one long, narrow rectangular street bar where the band was placed at the far end, that feedback was nearly uncontrollable - until I took one of our cabs and placed it a foot away from a long brick wall, and pointed directly at it. The other cab was placed near and at a 45 degree angle to the opposite brick wall, bouncing back at the band. This broke all the "rules" but it created an entirely reflected, diffused sound field in which the band could hear itself and standing waves that grow into feedback were minimized. The variable was the unusual, extreme room size and shape.

It it intuitive to me that a slightly rough inside surface will tend to dampen internal reflections of high or ultra high overtones an instrument can produce. Where those overtones reinforce lower frequency tones to create a harsh sound overall, dampening will be a benefit. Conversely, a hard, smooth and polished interior surface will tend to increase reflections of high frequency overtones.

While we might not be able to hear these as discrete sounds, we certainly will pick them up as high amplitude reinforced waves, albeit at a frequency we can hear. If the overtones reinforce a lower frequency waveform, we will hear the increased amplitude that results in the lower wavelength. It will change the nature of the sound we do hear even though the "offending" overtone is at a frequency we cannot hear.