Question for Luthiers: thinning top on a finished guitar

Construction and repair of Classical Guitar and related instruments
SteveL123
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Question for Luthiers: thinning top on a finished guitar

Post by SteveL123 » Tue Jul 25, 2017 6:08 pm

I like to tinker and learn with my guitar, not a concert level instrument. So far I have repaired a crack, converted the bridge from 6 to 12 hole, adjusted action by making a new nut and saddle, increased saddle string spacing from 55 to 57 mm. Work so far made the guitar much easier to play.

I made a Hacklinger gauge and took measurements of the top back and sides.

Here's a pic of measurements of the top, which ranges from 2.7 to 3.3 mm (thinner at the edge), the back is 2.6 mm and sides are 2.4 mm.

From my research, the top thickness is on the thick side. It's a solid cedar top, the bridge glue joint looks like it is lifted slightly on the bottom. After I buy another guitar (a concert level instrument, even though I've been told I am not ready for one :lol: ), I would like to remove the bridge, thin the top, do a better job on the repaired crack, fix the string burns and refinish.

Looking for advice and ideas on best way to do the above to make the guitar more resonant and sound better. It's pretty good the way it is but it seems to be very sensitive to to high humidity days where it would become a dud and not resonant at all. Will thinning the top make it more resonant and less sensitive to humidity?

These are the pertinent numbers on the guitar:

tap tone of top 109 hz A2 (grabbed top transverse bar, muted strings, tapped bridge, measured with elec tuner)

scale length 656 mm
Body length 490 mm

body width:
upper bout 289 mm
waist 240 mm
bottom bout 375 mm

body depth
heel 90 mm
bottom 100 mm

weight 1525 grams
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eno
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Re: Question for Luthiers: thinning top on a finished guitar

Post by eno » Tue Jul 25, 2017 8:48 pm

I'm thinking about doing the same on my Takamine C136, the top is very thick (I haven't measured it buy looks something like 4mm) plus it has a thick nitro finish, and as a resuld the sound is kinda dull. Another reason is that it has too high action (warped neck) that would need ~4mm of a bridge shaving. So I thought if I thin the bridge a bit from the bottom side and also thin the soundboard it will also help improving the action. I guess when thinning the top the soundboard would need to be tuned, right?
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Rokutaru Nakade No.9 1969, 1967
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James Lister
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Re: Question for Luthiers: thinning top on a finished guitar

Post by James Lister » Wed Jul 26, 2017 10:54 am

Provided you're sure the top is solid cedar, I'd certainly have a go at thinning it down. The body resonance is too high (as on most factory guitars), and if it has a thick lacquer finish, there could be much to gain from removing it, thinning the top, and re-finishing with oil (or French polish if you're brave).

You could do it without removing the bridge, which would have the advantage that you could check the change in body resonance as you thin down the top, and even put the strings back on as you go to see what progress you're making with the tone.

I'd aim for around 2.2mm around the edges, and 2.7mm in the centre, but stop if the body resonance goes much below G.

Be careful taking too much wood off near the rosette - there's no way of telling how deeply it was inlaid. I normally expect at least 1.5mm, but I sanded right through the outer rings of a rosette once which turned out to be only about 0.3mm deep.

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Re: Question for Luthiers: thinning top on a finished guitar

Post by SteveL123 » Wed Jul 26, 2017 2:11 pm

James,

Thanks for these excellent ideas! I care much more about sound than appearance on this guitar. I will leave the bridge on so that I can string it as I thin the top and observe (and make a video to record) changes in sound. If the bridge lifting issue gets worse, I can always remove/re-glue then.

In regards to not knowing the rosette thickness and possibly thinning through it, can I not thin the top above the rosette and leave as is? I've read that upper bout part of the top does not do much in producing sound.

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Re: Question for Luthiers: thinning top on a finished guitar

Post by James Lister » Wed Jul 26, 2017 3:23 pm

SteveL123 wrote:
Wed Jul 26, 2017 2:11 pm
In regards to not knowing the rosette thickness and possibly thinning through it, can I not thin the top above the rosette and leave as is? I've read that upper bout part of the top does not do much in producing sound.
Yes, there's no need to thin the wood at the rosette (or the rest of the upper bout) - just that if you're taking quite a lot off, you might hit the rosette just blending it in so that you don't have a "step" between the lower bout and rosette area. Shouldn't be a problem if you're careful.

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Re: Question for Luthiers: thinning top on a finished guitar

Post by SteveL123 » Wed Jul 26, 2017 4:36 pm

James, what tools would you suggest for thinning the top? Plane? Sand? Scrape? Or combination of all 3, if so, in what order? I can check thickness easy enough as I thin with the Hacklinger gauge so I don't go too thin.

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Re: Question for Luthiers: thinning top on a finished guitar

Post by SteveL123 » Wed Jul 26, 2017 4:44 pm

eno wrote:
Tue Jul 25, 2017 8:48 pm
I'm thinking about doing the same on my Takamine C136, the top is very thick (I haven't measured it buy looks something like 4mm) plus it has a thick nitro finish, and as a resuld the sound is kinda dull. Another reason is that it has too high action (warped neck) that would need ~4mm of a bridge shaving. So I thought if I thin the bridge a bit from the bottom side and also thin the soundboard it will also help improving the action. I guess when thinning the top the soundboard would need to be tuned, right?
Hi Eno, What year is your C136? I just ordered a 1977 C132S from Guitar center in Fort Wayne http://www.guitarcenter.com/Used/Takami ... adType^PLA and is being shipped to my local store

If it turns out to be a good one, I will keep it and have a guitar to play and start work on thinning the top.

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Re: Question for Luthiers: thinning top on a finished guitar

Post by James Lister » Wed Jul 26, 2017 5:23 pm

SteveL123 wrote:
Wed Jul 26, 2017 4:36 pm
James, what tools would you suggest for thinning the top? Plane? Sand? Scrape? Or combination of all 3, if so, in what order? I can check thickness easy enough as I thin with the Hacklinger gauge so I don't go too thin.
Definitely not plane. Scraper is best if you're reasonably confident with it. Of course you could probably remove the finish with something chemical, but I hate using chemical strippers, so I'd normally scrape first, then switch to sanding when I'm within about 0.2mm of my desired thickness. Either way, don't put too much pressure on the top, especially when you get close to 2.0mm.

James

P.S. you need to be particularly careful with the scraper on cedar as it's easy to pull chunks out if you're working across the grain.
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eno
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Re: Question for Luthiers: thinning top on a finished guitar

Post by eno » Wed Jul 26, 2017 8:01 pm

SteveL123 wrote:
Wed Jul 26, 2017 4:44 pm
Hi Eno, What year is your C136? I just ordered a 1977 C132S from Guitar center in Fort Wayne http://www.guitarcenter.com/Used/Takami ... adType^PLA and is being shipped to my local store

If it turns out to be a good one, I will keep it and have a guitar to play and start work on thinning the top.
Mine is 1976, pics are here
It actually sounds good, kind of "imperial" and nobel, but a bit dull and lacking high frequencies and harmonics/overtones. It's supposed to be a copy of Ramirez 1a, and I was wrong, the top is not 4mm but actually 2.7 like in Ramirez (it's the plate under rosette that makes it look thick). I guess it's just the way it sounds with cedar and finish. I plan to refinish with FP, hopefully it will open the sound a bit.
Paulino Bernabe 'India' 2001
Rokutaru Nakade No.9 1969, 1967
Masaru Kohno No.6 1967

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Re: Question for Luthiers: thinning top on a finished guitar

Post by SteveL123 » Thu Jul 27, 2017 2:43 pm

James Lister wrote:
Wed Jul 26, 2017 5:23 pm
SteveL123 wrote:
Wed Jul 26, 2017 4:36 pm
James, what tools would you suggest for thinning the top? Plane? Sand? Scrape? Or combination of all 3, if so, in what order? I can check thickness easy enough as I thin with the Hacklinger gauge so I don't go too thin.
Definitely not plane. Scraper is best if you're reasonably confident with it. Of course you could probably remove the finish with something chemical, but I hate using chemical strippers, so I'd normally scrape first, then switch to sanding when I'm within about 0.2mm of my desired thickness. Either way, don't put too much pressure on the top, especially when you get close to 2.0mm.

James

P.S. you need to be particularly careful with the scraper on cedar as it's easy to pull chunks out if you're working across the grain.
I have used all these woodworking tools in the past but not a lot of practice recently. I will practice on scraps before touching the guitar. After scraping the finish off, isn't a small finger plane much faster in thinning than scraping or sanding? If I take very light cuts, is there still a danger of pulling chunks off if going against the grain (which I shouldn't be doing)? Since the bridge will be there, I'll have to make sure the plane does not collide with the bridge.

What would a shaving from a scraper on cedar look like and what thickness should the shaving be approximately? Do you know of any videos on scraping cedar?

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Re: Question for Luthiers: thinning top on a finished guitar

Post by Douglass Scott » Thu Jul 27, 2017 3:01 pm

Like James said, stay away from planes including small finger planes. It's asking for a very uneven, unpredictable result since it's an awkward surface - flexible with stiffer ridges where braces are. Maybe you can find a good video about card scraper sharpening and what it looks like to use a really sharp scraper. I'd choose a thick scraper - 0.75mm - 1mm thick. After sharpening, cover all four corners of your scraper with a little piece of masking tape so the corners cant dig into your top, and to help fare the cutting action down to 0 toward the ends of the scraper.

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Re: Question for Luthiers: thinning top on a finished guitar

Post by SteveL123 » Thu Jul 27, 2017 5:42 pm

OK, point taken in not using a plane. Wouldn't tape on the corners of the scraper make the scraper sit higher, plus it will wear through with use (if I don't keep an eye on it) then the corners will dig into the top? How about rounding the corners with a grinder instead of tape?

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Re: Question for Luthiers: thinning top on a finished guitar

Post by Douglass Scott » Thu Jul 27, 2017 6:09 pm

SteveL123 wrote:
Thu Jul 27, 2017 5:42 pm
OK, point taken in not using a plane. Wouldn't tape on the corners of the scraper make the scraper sit higher, plus it will wear through with use (if I don't keep an eye on it) then the corners will dig into the top? How about rounding the corners with a grinder instead of tape?
Making the corners of the scraper sit higher is exactly the idea! So the scraping action is progressive from 0 at the tips. Masking tape is only .005" thick so you'll hardly notice it, and you'll often scrape with one end of the scraper hanging over the edge of the guitar so it's only the end closest to the bridge that isn't cutting. I find the tape lasts pretty long, but certainly keep an eye on it!

Rounding the scraper corners as well is a great idea. I have done exactly the same on scrapers that I use for soundboards. Nonetheless just one layer of masking tape at the tips is still helpful, especially while you're learning.

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Re: Question for Luthiers: thinning top on a finished guitar

Post by SteveL123 » Thu Jul 27, 2017 6:15 pm

Douglass Scott wrote:
Thu Jul 27, 2017 6:09 pm
SteveL123 wrote:
Thu Jul 27, 2017 5:42 pm
OK, point taken in not using a plane. Wouldn't tape on the corners of the scraper make the scraper sit higher, plus it will wear through with use (if I don't keep an eye on it) then the corners will dig into the top? How about rounding the corners with a grinder instead of tape?
Making the corners of the scraper sit higher is exactly the idea! So the scraping action is progressive from 0 at the tips. Masking tape is only .005" thick so you'll hardly notice it, and you'll often scrape with one end of the scraper hanging over the edge of the guitar so it's only the end closest to the bridge that isn't cutting. I find the tape lasts pretty long, but certainly keep an eye on it!

Rounding the scraper corners as well is a great idea. I have done exactly the same on scrapers that I use for soundboards. Nonetheless just one layer of masking tape at the tips is still helpful, especially while you're learning.
Ok! Maybe I'll round one side and try tape on the other. I am not sure where my scrapers are or if I can find them! I bought a set of different shapes many years ago. They are somewhere but I don't know where atm. What and where is a good scraper to buy?

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Re: Question for Luthiers: thinning top on a finished guitar

Post by simonm » Thu Jul 27, 2017 7:12 pm

When you get to the sanding stage, you may find that the dust is unbearable. Red cedar dust is nasty stuff. I strongly recommend a mask and/or working out of doors. Cedar is also very prone to damage. Just looking at it seems to put dings in it.

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