Please help: neck reshape: yes or no

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Please help: neck reshape: yes or no

Postby jorge rodriguez » Tue Apr 02, 2019 10:13 am

Hi all,

I've been a member of the forum for a while but it's been long since I last posted something. I have a doubt regarding acoustic guitar modification and I hope someone can help.

I am an amateur builder and repairman. This is not my way of living but I learnt building with two different luthiers here in Barcelona, Spain, and have since built a few instruments. One of them is an acoustic dreadnought guitar that I built about five years ago. This instrument has a nice big, bassy sound and plenty of volume and I love it. The only thing I would change now is the neck profile. It is kind of U shaped and feels too big. It is playable and I've always played it as is, but I can't help thinking that with more of a D profile it would be a lot more comfortable. So I keep thinking about reshaping the neck and removing some wood from the shoulders of the neck. I don't think it would be necessary to remove a lot of wood.

So my doubts are basically two:

- Would the modification change the guitar sound/tone/volume? If there is any chance that the change affects negatively I would probably not do it. It is a very special guitar for me, so I don't want to risk it.

- The finish is nitro. If I wanted to do it, would I have to remove the finish from the whole neck?, or would it be ok to remove it only where necessary and then refinish the affected parts? The finish is transparent, it has no stain or pigment.

I know there is no 100% sure answer to the first question, but I would like to know the feedback from anyone with experience in the subject.

Thanks in advance!

Jorge
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Re: Please help: neck reshape: yes or no

Postby Barry Daniels » Tue Apr 02, 2019 11:35 am

I would not try to save any finish on the neck. Carve it to your desired shape, and sand it to blend the shape together. There will probably be little finish left after it is properly shaped and sanded.
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Re: Please help: neck reshape: yes or no

Postby jorge rodriguez » Tue Apr 02, 2019 11:36 am

Thanks Barry! I will keep in mind if I do it.
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Re: Please help: neck reshape: yes or no

Postby Freeman Keller » Tue Apr 02, 2019 11:44 am

I have reshaped the necks on two guitars after they were finished. Both were too "chunky" - its hard for a new builder to remove enough wood. In both cases the guitars were finished, I had to refinish the necks basically from scratch. Both guitars had bold on necks and I removed them to do the work.

Here is a slot head 12 string -

Image

Image

Image

- I don't believe the mod will change the sound of the guitar. Some people think neck mass does have an effect, I think it will be slight.

- Nitro is pretty easy to fix. You'll probably have to go thru whatever pore filling you previously did and you may have lines between the old and new finish. Might be easier to just strip the whole thing and refinish.

- Remember that you have a truss rod lurking somewhere inside. Don't get too close to it.

Otherwise I say go for it. I hated my guitars before I modified them, they are much better now.
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Re: Please help: neck reshape: yes or no

Postby David King » Tue Apr 02, 2019 12:13 pm

If you have a steel truss rod you can get a good sense of how deep it is in the wood using a rare earth magnet. I use a round magnet and let it roll back and forth over the back of the neck. The faster it bounces back and forth the closer you are to the truss rod underneath. If you want to stop carving 3mm from the rod then see how the magnet behaves over a similar piece of steel with a 3 mm thick piece of wood over it.
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Re: Please help: neck reshape: yes or no

Postby John Clifford » Tue Apr 02, 2019 12:25 pm

I agree with the other comments. If you don't have a bolt-on neck, then you'll need to think through how you are going to deal with the neck heel area, both in reshaping and refinishing. If you end up removing a significant amount of wood, there is the possibility that the neck will become less stiff, which could result in some damping of the strings, meaning loss of sustain and perhaps slight tonal difference, but I doubt the effect would be very noticeable. In an extreme case, it could affect the way the neck bends in response to string and truss rod tension, but it doesn't sound like you'll be taking off that much wood. If you like the guitar, but the neck shape is bothering you, I agree you should go for it. If nothing else, you'll gain experience.
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Re: Please help: neck reshape: yes or no

Postby Alan Carruth » Tue Apr 02, 2019 12:35 pm

Neck stiffness can, in some cases, affect the sound, but in this case I doubt it. If the neck is stiff enough, and the headstock light, lowest vibration mode of the guitar (sometimes called the 'neck' resonance) can be as high in pitch as the 'main air' resonance. When that happens to two modes couple. The 'air' resonance gets split into two peaks which are lower than usual, but the area under the spectral curve (the 'total available horsepower') is increased. The lowest notes tend to be come 'fuller' and 'darker' and there is less chance of having a 'wolf' note at that pitch, which is often around G on the low E. Normally the 'neck' mode is quite a bit lower that the air mode on steel strings; often around C below the low E. The skinny 14 fret neck is flexible, and the tuners can be heavy. The match is not uncommon on Flamenco guitars, particularly ones with pegs; the neck is shorter and beefier, and the head light.

There are other, higher order, modes where the neck bends a lot, and removing wood will alter the pitches of those as well. They are not as active as the lowest pitched mode, and there is no easy way for them to couple with body vibrations in a way that can produce sound, so they usually don't have much of an effect. It's also much harder to predict the pitches. It's just possible that if you're playing a note at exactly the pitch of one of these resonances, AND the fret happens to be in a spot on the neck that is really active for that mode, that sound from the string could be diverted into the neck rather than driving the bridge and producing sound. You see dead notes from this on solid body guitars, and particularly on solid body basses, where the neck is the most flexible part. On an acoustic, where the top is much easier to move, it's far less common: I can't say I'ever seen it, but it gets brought up by folks with a lot of experience with solid bodies once in a while, and the theoretical possibility has to be acknowledged.
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Re: Please help: neck reshape: yes or no

Postby jorge rodriguez » Tue Apr 02, 2019 4:57 pm

Thanks everyone for your replies. It is very useful information for me.

I am not worried about the truss rod, as I wouldn't be removing any wood (or very little) from the center of the neck, as I think thickness is ok. It's only the shape that I don't like, and removing only some wood from the shoulders would be enough.

I am worried about working at the heel, though. It is a glued dovetail joint, so removing the neck would be too much trouble. If I have to remove all the finish from the heel area it would be difficult without de-attaching it from the body. So I just wondered if it would be a good idea just to remove the lacquer from the areas where I need to remove wood, sanding afterwards to kind of "merge" the bare wood with the old lacquer, and refinishing all the neck later. Not sure if I am explaining it properly, or if it is a good idea. What do you think?

About the tone, I assume it might change a little bit, but if it's unlikely that the change is drastic , I might go for it.

Jorge
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Re: Please help: neck reshape: yes or no

Postby Brian Evans » Wed Apr 03, 2019 10:30 am

I'm actually about to do this to two guitars, both are early versions of necks I thought I liked, but didn't. Both also are victims of using hardware store acyrlic lacquer, which is happily shedding off randomly - so full refinishes are to follow. I would take all of the finish off the back of the neck, preserve the headstock finish if it has exceptional inlay, and tape off using hard corners to separate the new from the old. That might mean taping the sides of the body tight to the dovetail, and the face of the headstock. Nitro can blend in, but I might be more worried about matching colour exactly, after so many years.
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Re: Please help: neck reshape: yes or no

Postby Freeman Keller » Wed Apr 03, 2019 11:25 am

Jorge and Brian, one thing that I have learned about making necks is to don't trust my hands to tell me its the right shape. I make a point of making at least two templates off of a neck that I like and carve to fit them. You can buy a clever little device (I think its for duplicating moldings) that makes the task easy.
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Re: Please help: neck reshape: yes or no

Postby jorge rodriguez » Wed Apr 03, 2019 4:30 pm

Thanks Brian and Freeman.
Yes, I have one of those devices, Freeman, and I will use it to copy the profile of another neck that i particularly like.
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Re: Please help: neck reshape: yes or no

Postby Chris Reed » Thu Apr 04, 2019 4:31 am

I'd expect the neck wood to have changed colour because of light exposure, and so I'd remove all the finish on the neck shaft (a) to get a uniform colour, and (b) to avoid visible lines where new and old finish meet.

However, I think you might get away with feathering new finish into old at the heel. There's a lot of end grain at the heel, so it's probably a little different in colour from the shaft anyway, and the compound curves at the heel/shaft transition should help reduce the noticeability of your feathering.
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Re: Please help: neck reshape: yes or no

Postby jorge rodriguez » Thu Apr 04, 2019 6:42 am

Thanks Chris, that is interesting information.
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Re: Please help: neck reshape: yes or no

Postby Randy Roberts » Fri Apr 05, 2019 8:36 pm

AS far as templating the shape of your reshaped neck, you might want to consider this stuff....

viewtopic.php?f=12&t=5838
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Re: Please help: neck reshape: yes or no

Postby jorge rodriguez » Sat Apr 06, 2019 7:26 pm

Thanks Randy, that looks very useful.
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