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Hi all, before I get mercilessly trolled for this you should know this is only my 3rd guitar build!

Long story short I am loosely following the Cumpiano book and when reading the back bracing measurements for the lengths I got them the wrong way around, cutting the 1/2” x 5/8” brace down to 14” instead of 18”.

I had no more bracing stock so decided to instead cut a 45° mitre on the horizontal plane on the two pieces and stick them back together with CA glue.

When doing a test fit of the back I heard a crack while I was confirming the back to the arch of the sides but couldn’t see where it came from.

Not sure how I missed it but now that the back is glued on I noticed that the mitre join has failed.

My question is, what glue would you use on the repair? I’m guessing that because the CA was applied to end grain it absorbed a bit and made for a weak joint, therefore Titebond is probably the way to go?

Also in absence of a scissor jack I’m thinking of centre drilling two short lengths of some 30mm oak dowel I have, running some threaded rod through the centre of two pieces so it looks like a dumbbell and using wing nuts to expand the jack. I was planning on lining the two ends with cork but still have concerns that the lower face brace on the top which is the closest opposite brace to the failed joint is quite scalloped and therefore maybe not the best thing to push against to close the break.

Am I better off making a cork lined caul that sits on as big an area of the soundboard as possible opposite the break maybe?

Thanks in advance.


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Can you remove the back and make a new brace? I can send you a piece of brace stock - either spruce or mahogany - so you can do it right. Futzing around inside a closed box is possible but a real PITA - if you haven't bound it yet I would take it apart.

The other problem with doing this repair is that I'm not sure how well Titebond will adhere to the CA that you've got in the failed joint.

I don't think you can glue this joint successfully. You don't have enough glueing surface, and end grain joints won't take structural stresses. You might have got away with a long splice, 10:1 or more, maybe.

I'd remove the back and replace the brace. I can't see any other answer.

Not ideal, but you could add on a scab brace to the side which covers the splice.

If it's only just on, there's no binding to worry about, take the back off (heat-domestic iron? and a thin blade) and replace the brace. brace is U/S.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Cap it on the top like you do with an X-brace joint. A 1/8" thick piece that is at least 1" overlapping the joint on both sides.

Clamping wise - make yourself some little go-bars out of popsicle sticks and wedge them in place.

In the future - you can get spruce brace stock from dimensional lumber - you just have to sift the pile.

I wouldn't spend time trying the repair. A 45 degree scarf, even if done with the right glue and full access, is not likely to survive long.

As a 3rd guitar, why not just keep going? As likely as not, it will still be your best sounding/functioning yet. If it rattles when you're done, you could practice the task of removing a brace cleanly through the sound hole and just leave it out.

How does the sound change with it out? better? worse? The same?

If the back seems vulnerable, you just want it in there, or want to learn how to glue one in through the sound hole, make a new one and only then give it a go.

Just my .02 on an option.

More heresy:
Buy a light 2x4 or pine board and try splitting and sawing some braces out of it. A small amount of added height would easily compensate for it's lower class origin story.
Premium back brace material may be relevant at the very highest levels of building but that's unlikely on an early instrument in your long and successful career!

If you are not too concerned about looks you could gob some PL400 construction adhesive in and around the joint. You could scab some small thin pieces of wood on either side of the joint and press them into the adhesive. It wouldn't be pretty , but it should hold it.

Premium brace wood is something you can find in a dumpster. I look for old construction grade spruce 2X stuff that looks like it has been laying around long enough to dry out some. When splitting out brace stock you will waste a lot of wood. You will some "premium" braces and a pile of kindling. Not all of the "brace stock" you buy will make high quality braces.

Thanks guys. Here’s a photo of the offending brace.




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It looks like you are doing nice, clean work. I would take the time to remove the damaged brace and do it as you originally planed. You won't regret it.

Since you used CA you are pretty much stuck with it if you do try a repair. Without knowing why it failed the first time it will certainly fail again.

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Joe Beaver
Maker of Sawdust

No one has asked you what kind of perfect this guitar is yet. Is it a real beauty? Can you sell it for 4K?
If the answer is no, here's a a good easy fix. Make that tool with a small turnbuckle and a couple of hunks of wood, slo CYA the joint and use some wax paper to have a release. Let that cure overnight and then sand if you can reach it. put two short 3" braces front and back of that brace, centered on the joint so that each covers the area 1-1/2" each side of the joint. They don't have to butt up to the brace with a joint, can be a few 16ths away, even. Use yellow glue IF you have not slopped CYA all over the back in that area. Use cleats too if the back is cracked also.
Hope that makes sense to you...won't change anything tonally.

Next time taper your end block top and bottom like this, right up to the thickness of the kerfing...

Hi Joe,

Thanks very much for the kind words.

Can you elaborate on the CA glue comment? Are you saying now I’ve used it once that’s what I’m committed to, and therefore can’t / shouldn’t try Titebond?

My reservations with replacing the brace are twofold at present regarding removing the back.

Firstly, the binding is installed on one side and more importantly the channels are routed including one for a purfling strip on the inside of the binding. This is installed. I just worry that I am now dealing with extremely thin timber in the routed channels.

Secondly, I know exactly when and why it broke. I was test fitting the back and started at the neck end. By the time you reached the tail the back was significantly raised. In trying to work out how to make it conform I just applied a bit too much pressure on the back and I heard the brace give but for some reason couldn’t see what had made the sound, despite the culprit being an obvious one. You live and learn!

After a few more gentle attempts I realised that if I started from the tail and worked up to the neck end it was a lot easier to gently conform the back to the sides.

For these reasons I’m tempted to try gluing and maybe adding a spline or similar as suggested above and then working on the neck for a bit. I’ll then apply some reasonable pressure on the problem area and see if the brace gives. If it does I’ll have to attempt taking the back off.

The project it going extremely well otherwise and I want to minimise the chance of destroying it at this stage, especially as I’m building this one for a friend.

Cheers, Tom


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Titebond will not work in the joint because it will not stick to CA. But CA has the ability to stick to cured CA.

I second Haan's approach.

I third Haan's approach.

Ok great, thanks guys. FYI the project is a Grand Auditorium style steel string with a cutaway. Torrefied spruce top, walnut back and sides. Will it be a $4000 guitar? Nope.

I guess what I like about Haan’s approach is that it’s a lot less invasive than taking the top off, and really nobody will see the repair unless they go looking which the intended owner won’t. I’m only hanging him for materials anyway.

Maybe I need to charge extra for the additional braces...


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"Maybe I need to charge extra for the additional braces.."

Now you're talking like a Pro..... It's a Feature, not a Mistake!

Yes it is an experimental bracing pattern to improve base response!!!!!!

A 8:1 scarf joint (12:1 better!) would provide the same stiffness and decent longevity, but attempting to do that work inside the body would likely require a good video camera, some purpose-made tools, and a good deal of skill. If the goal is to maintaining the stiffness profile across the joint you expect to fail (a 45 degree end grain to end grain joint is little better than a butted joint) to prevent what will likely generate a cracked back plate over time, a second, partial length brace behind the offending back brace (sized at the same width and height, and 24 times the height in total length, with taper to zero height) would work. Still, this is a second-rate repair at best compared to replacing the brace entirely.

I think the suggestion made earlier in the thread - to remove the back - is wise as it allows you to resolve the problem with a new replacement brace glued to a clean, glue-free surface. It may also let you address the other issue seen in the photo - what appears to be significant scallop or excessive taper in your upper X brace arms. Scalloping above the X or excessive taper can result in excessive deformation of the top in front of the bridge, as well as the potential for sinking of the back edge of the soundhole and at the edges of the waist just above where the upper X arms land.

Of the two issues, I am more inclined to consider the upper X arms as a reason to pull the back than the back brace, but if the X braces are truly an issue versus just a photo artifact, pulling the back allows both to be addressed.

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A constellation only takes shape when one maps the whole.
- Beth Brower

Hmmm that’s a concern. I did scallop above the X. Let me know your collective thoughts on whether you think this will be an issue, and can I ask that only those professionals with direct first hand experience throw in their 2c here so I can get a clear and wholly objective set of feedback?

Here’s my thinking; if it is an issue I’ll take the top off rather than the back because I’ve already fitted one side of binding on the back and, more significantly, routed the binding channel. The top will be much easier to remove. My question is, how do you release the soundboard at the head and tail block without affecting the centre seem? Same for the x-brace.


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Ouch!

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Ouch indeed. With every balls up I learn a new skill though. I’ve just ordered a separation knife...


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Not speaking as a professional (having never made my living as a luthier) but as a hobbyist who has for the past forty couple years built and repaired musical instruments, the scalloping of the upper arms of the X brace, although not ideal, is probably not a fatality. The tighter waist of the Grand Auditorium will add some support to this area and the X brace is by design a very strong construction.
If you do want to add some additional support to this area you could glue some braces behind and parallel to the upper arms of the X (similar to what Haans has suggested for the back brace). This could easily be done through the sound hole using small "C" clamps.
Removing the top (aren't torrified tops known to be somewhat brittle?) or the back of a guitar, unlike a violin is not commonly done unless it is going to be replaced with another.
The perspective of a Pro might be different - he not only has to make it work, but it also has to "look right".

How thick is the top?
Put a cap on the X brace...
At some point you have to ask yourself, "Is it worth it all to rip off the top and toss it, rip out the back brace and replace...or just fix a couple things and move on". I think fix and move on. You need experience more than a perfect guitar...that will come later.

Bosco, limiting your questions to "only those professionals ..." might only alienate some who have the knowledge and skills to help answer your questions. Use your judgement to decide which responses might work best for you.
Someone with only a couple under their belts might have come up against this same problem, and made an effective repair!

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

That’s a fair point. I guess I just wanted to ensure that anyone comment was basing their suggestion on real life experience rather than what they read somewhere on a forum once upon a time.


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