acoustic guitar build problem

[Justin Tugaoen]

Hello. I started a couple acoustic guitars in 2011 and got as far as getting the body all glued up with back and tops. So now I'm getting back to them again to start binding, but noticed on one of them that the back is no longer domed with the original 15' radius. It's about a grand orchestra size, spruce top, figured narra back and sides, Martin OM-type bracing. Looking at it closely, the area between the headblock and tail block has flattened out (the longitudinal radius), where there's a noticeable bump where the headblock is. The movement has caused a small longitudinal crack about 3 cm to the right of midline close to the tailblock. Looks like there's still some crosswise radius by the middle crossbrace, and the lowest crossbrace. It's quite flat at the 2nd from the bottom brace, which is a wide, but short brace. Any thoughts? Thinking back I'm starting to wonder if I used titebond II, which I know has some creep, but not sure. So do you think I should take off the back and re-do the bracing (I'd rather not, but I will if that's the best long=term solution); or should I leave it as is; there are such things as flat-backed guitars, arent' there?
btw, I've since glued up the crack, and sanded down the bump so the bump is no longer visible. Overall it looks good and the flatness isn't noticeable without looking at it closely. Thanks for your advice!

[Chuck Tweedy]

It sounds like you built the body at high RH (or the wood was still not fully seasoned).
Since it is an unbound body, I would remove that back from the body and braces from back, get it dried out properly, then re-attach.

[Justin Tugaoen]

I'm not sure the RH was the problem. After buying the wood, I kept it stored in a dry environment for over 5 years prior to using it, and my shop stays pretty dry. Regardless, it happened. So is it a big issue going forward if I don't re-do the back? Also, what's the best way to remove the back, glued on with titebond?

[Doug Shaker]

OK, let's reason this out.

Without an arch, the back will be weaker and less stiff. The soundboard will need to do more to resist the pull of the strings and the bridge may pull up easier.

The back will also be floppier. This will make it less resonant more likely to absorb sound energy rather than reflecting it back to the top or radiating it as sound. So the guitar won't be as loud.

But that's just guess work. What are the two paths that are open to you?
1) Finish the build as is and take the risk that, if you don't like it, you will either have to give the guitar away to someone who won't care or pull it apart, reglue and refinish it.
2) Pull the back off, pull the braces off, and reglue everything.

If it was me, I would go with #2. I'm not trying to learn to make second-rate guitars. I'm trying to learn to make superb guitars. That means, while I am learning, I do a LOT of rework. All the time, I'm thinking, "Well, this will teach me."

If it is Titebond I, steam should loosen the glue.

[Barry Daniels]

I would remove the back with a household iron. Just go around the edge and use a putty knife to separate the joint after it's hot. When you get near the center joint be careful not to overheat it.

[Mario Proulx]

What Barry said. Since there isn't any finish on it yet, the iron will work fine. Start at the lower bouts, and work toward the neck and end blocks, then work those. Chisel-off the braces, and start over.

[Justin Tugaoen]

Ok, sounds unanimous although the idea of having to remove the back is a little intimidating. So once I remove the back, do you think I'll have to do anything to it before re-bracing? Meaning, will the back be otherwise flat once I remove the braces? Or will it be wavy/warped? One other thing I'm worried about is being able to re-glue so it fits the sides properly, since I've already trimmed the edges to the exact shape; I guess I'll find out! Thanks again for your inputs.

[Barry Daniels]

The back should be flat when the braces come off. If not, weight it down.

Placement of the back onto the sides has a little wiggle room because you will route for the bindings/purflings.

[Rodger Knox]

Do you have an outside mold? If not, it would be worth the effort to make one. I'd trace the back onto 1/2" plywood, cut it out & slip it on before removing the back.
Don't be intimidated, the hardest part will be getting the back reglued in the same position after it's fixed. With a mold holding the sides in place, that shouldn't be too difficult As Barry said, you have a little wiggle room. If the back wasn't already trimmed, it would be a really simple fix. Getting the back off should be easy. It may take a little time to get it to start to release, but once you get titebond hot enough it will come off pretty quickly.

[Justin Tugaoen]

Great, thanks a lot for the advice. Yes I do have an outside mold, so I'll use that

[Doug Shaker]

Yes! Put everything in the mold. With the back off, the sides will want to pop open and change shape. If you don't have it in a mold, it will be hard to fit the back on again.

But, if the back won't fit on again, you can make new wider linings and wider bindings.

[Michael Lewis]

Justin, I have a question regarding your sides. Are they contoured for the arching or are they just cut to a pattern and glued on?

In order to maintain the 'dome' effect of the back plate the sides have to accommodate that arch. Also, just before you glue on the new back braces run a hair drier over the back and quickly glue the braces in place. This slightly shrinks the back across the grain, sand as it picks up moisture over the next day or two will help reinforce the cross arching. You are using a hollow form for this aren't you?

[Justin Tugaoen]

Hi Michael, yes I did use a hollow form, for gluing on the braces and for sanding down the sides and head/tailblocks to match the 15' radius prior to gluing on the back. It looked fine after the glue-up, but I guess the back shrunk some since (couple years); not sure how, since that wood's been in the shop for a few years now, and my shop is in my finished basement with fairly controlled humidity (currently ~52%, maybe as low as 40-45 in the winter). Maybe when I take it off I could try an x-bracing pattern for the back?

[Michael Lewis]

I wouldn't X brace it, just make new braces with the radius and proceed with the hair drier and glue up. A proven design will work if properly done.

[Justin Tugaoen]

Sounds good. That's what I'll do

[Justin Tugaoen]

Well, thanks to you guys I went ahead and took off the back of my guitar (with much trepidation) and it surprisingly came off fairly easily, but with a couple issues, including a huge 4" crack while I was removing the braces and separation at the midline by the endblocks from overheating with the iron (although I did try to be careful). With the back off I noticed other more minor, superficial cracks which I repaired with ca glue. The larger cracks looked to have been under a lot of tension because they separated as much as 1/8"; the midline cracks I couldn't approximate closer than 1/16" so I repaired those with ca glue and overlapping wood inlays on the back. The other cracks I had to glue on some cleats. Reminds me of the restoration articles I've read in the GAL journals! This wood is definitely not as stable as other woods I've used, so I made the braces just little bigger and taller this time around.
Interestingly, before removing the back, I laid the guitar body down on the 15' dish and was surprised at how poorly it matched; the waist sections were up to 1cm higher instead of touching the form all around. However, once I had the back off, I checked the shape of the sides against the form and it was nearly perfect. The back had warped quite a bit I guess. It was still a little wavy with the old braces off, but now looks really good again. So just a little more sanding and a new label and I'll be ready for glue-up. All in all, I'm so glad you guys pushed me to do the right thing; it wasn't as difficult as I imagined it to be, and definitely will be worthwhile considering all the work I've already put into this and the work I still have going forward. Can't wait to finish it now!

[Schennelly Stoughton]

I had the same problem with a walnut dread I built , I left it as it was , I suspect my bracing wasn't tall enough. The walnut was 40 yrs old and seasoned properly.It's the best sounding guitar in my collection.