Official Luthiers Forum!

Hello,

I have a friend who forgot to put any glue between the lap joint on the x-brace. He was distracted at an inopportune time. He was thinking about just wicking some CA into the joint after carefully masking off the soundboard.

Question:

1) Will this be sufficient from a stability perspective? It seems like it would be fine.
2) The main concern is with the possibility of distorting the color on the soundboard. If some did get through the joint and onto the soundboard on the backside, would it show through?

Thanks for any help for my friend. I think Robert O'Brien knows him.

Brad

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

CA will work just fine -- if the joint is not tight fit perhaps medium viscosity would be best. CA discolors when the glue enters the end grain so in this application there its not an issue. And normally when the stain is a concern we simply apply a sealer -- I use nitro lacquer. As a side bar, years ago like thirty! I glued all the braces in a classical guitar using CA -- no stains and that guitar is still intact today.

_________________
Ken Cierp

http://www.kennethmichaelguitars.com/

Thanks, Ken! Of course the joint in tight! Ha ha.

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

If you were to hold it soundboard up and wick in with a pipette it could be less messy.

I will do that. Thanks, Danny!

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

Martin style. Round piece of cotton t-shirt like material soaked in tite bond. Very messy to fit, masking helps. So do tooth picks. Etc. But once that's in place, no worries. I love that process. It would not matter at all about the lap joint faux pas.

And no. I would never trust CA for this task. Never. You want some amount of flexibility, and longevity. I wouldn't trust CA for that. To paraphrase Todd, The more I build, the less I reach for the CA.

Hi Mike,

After you posted this I did some searching for examples of this approach. I know that Martin did this now or in the past based on conversation I've read here on the OLF. Searching here I found information on using fabric for sides instead of side supports, would it be the same process? Have you seen any steps online on how to achieve this?

Thanks!
Brad

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

Scratch that. New search terms and I found an old post from you with details.

Thanks!
Brad

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

Gluing on a linen patch ala Martin does very little for the joint except cover it up
Using t shirt material would do even less.
I don't expect any adhesive to achieve much in this endgrain to side grain situation, but with a tight already assembled joint, Ca would be my choice, followed by a wood cap if possible.

Thanks, Jeff. I do indeed put the cap on typically, but I have just been doing a strip that covers the opening of the gap. I've seen others do a round disk like feature. Do you think some CA and then just a strip like I describe would be sufficient in this situation?

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

Absolutely,
If you cover the lap with a timber strip it is the strip which is restoring the strength and stiffness lost by the notch and the CA will just avoid any unglued contact at the lap

actually that linen does a lot more than you may think . In 20 yr of repair work I have seem a lot more wood patched fail than the linen. this isn't a beam it is a brace. I doubt that it matters all that much as the brace being glued to the top is what matters most.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

A wood patch will fail if it does not have enough gluing surface each side of the joint.
A linen patch may not "Fail" in that it does not separate or break, but this is because it is flexible and does not contribute to the stiffness of the joint.

Some Gibson "X" braces were not notched, but rather had two braces butted against a third longer brace. They seemed to work O.K. Has anyone done a bracing design with a single leg of the "X"? Maybe one diagonal leg would be enough to stiffen the area below the soundhole and above the bridge.

It would be interesting to try. I'm working on one with only the upper transverse and X:

But I think it would actually work with upper transverse and half of the X. The total stiffness would probably need to be higher, and obviously would be less evenly distributed. But the asymmetry might give it an interesting tone.

I've used that cotton patch since my start in this business. Of course, the lap joint is glued. Unlike what the OP is dealing with. I like the feel of it. The engineering makes sense to me. Could it fail? Anything can fail. I don't use the same idea for side braces. Mainly because I like the look of the wood braces. And, I'm not in a hurry. Anybody who has tried this will tell you it takes technique to get it done with minimal fuss. Toothpicks!

I do think given the OPs situation, this is a better solution than super glue.

First old Gibson I worked on I removed the patch and said "now I know what the patch was for, to cover an ugly joint" The notches were cut in triangle-shaped braces then little wedges were glued in to fill the void.

Edit: for those who haven't had the viewing pleasure. From a '49 J45

The guitar was in such poor shape it pretty much self destructed so made measuring braces and such pretty easy. Reassembly was not so easy.
Easy to see how the patch was trimmed and glued.


Wedges to take up the slop. They're still there so I guess they worked.

_________________
Steve Smith
"Music is what feelings sound like"

A few years ago I did some deflection testing on an X brace on support bars about 350mm apart. centrally loaded.
First was a single bar, diagonally across the supports, when loaded it deflected- 1.21mm
Then I added the cross bar by butting two short legs to the centre using gel CA deflection measured -0.74mm
I then added a very thin (0.75mm) wood cap on the underside also with CA deflection measured - 0.62mm

My conclusion was that an X brace with a well fitted glued joint and a wood cap is twice as stiff as a single brace.