Crack repair posssible?

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JurgenV
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Crack repair posssible?

Post by JurgenV » Sun Jul 17, 2022 4:06 am

Last year I started building a 000 size with a really nice Padouk set. somehow I didn´t find the time to finish closing the sound box with the top and left it for some month in the basement. Humidity and temperature are not controlled in my work space and I know that it changes often a lot. When I finally had time to continue the build I saw that the back had a crack following the interface of the sap wood (left side of the white area). I hope you understand what I mean. You can see it quite clearly in the picture.
As this time I have a customer who explicitly wanted this set of Padouk for her guitar: Do you see any chance of a repair?
I can take the back off again and plane the bracing. Will I loose too much of the back while routing off the binding? Do I need to insert a back strip to gain some width? And most important is it possible to reliably repair the crack without being visible. The guitar will be shipped to a region with much higher humidity than where it´s build.
Would be grateful for any advice
1657993446048_resize.jpg

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Mark McLean
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Re: Crack repair posssible?

Post by Mark McLean » Sun Jul 17, 2022 1:46 pm

Hi Jurgen
Most of us have experienced something like this. Humidity issues are a serious concern when you are building with solid wood, and unless you manage the humidity during certain stages of the building process you will get issues like this. It should be fixable.

First lets go over what caused it. Sorry if you know this already - but other people will be reading this also, so it is worth explaining. I am certain that this happened because you performed one or more of the critical stages of the construction at a time when there was high humidity. The plate expands in width (only slightly, but significantly) when it absorbs water from the atmosphere, and it shrinks again when it dries. Imagine if it is humid on the day that you glue the back onto the rims. It all goes well and looks great. Then, maybe 6 months or 6 years later, the guitar experiences dry conditions. Moisture content escapes and the plate wants to shrink by maybe 1 or 2%, but the outside edges are firmly attached to the rims and cannot contract. If you have built it with some arch in the back the first thing that you will notice is that it goes flat, because the arch is there to provide some wriggle room for expansion and contraction. But if it keeps shrinking, something has to give. It will usually crack at the weakest point - which might be your centre joint, but in your case it is the natural weakness where the sapwood joins the heart timber. To prevent this you need to avoid high humidity when building, and avoid it getting dry later. Ideal conditions for building are at Relative Humidity (RH) of 40-50%. Factories doing high volume production keep the whole production environment in this controlled humidity range all of the time. For people like us this is a difficult thing to achieve. In reality there are only 2 critical stages of guitar construction where it makes a big difference. These are the day that you glue up the braces, and the day that you glue a back or a top onto the rims. So, you can get by without fancy climate control if you just pay attention to the prevailing RH, measure it carefully right in your workshop, and choose the right time to do those important steps. Just make sure you do those jobs when RH has been stable at 40-50 for a few days. Don’t glue those steps when it is humid or rainy. You can do just about any other part of the build, just NO BRACING, NO CLOSING.

OK, so you broke the rule, and now the damage is done. Can it be fixed? The good news is that nearly everything can be fixed. You have a few options, going from the simplest to the most complex in order:
1. Rehumidify the guitar. The crack will probably close and you can glue it, cleat it or glue a joining strip inside, and it will look OK. BUT - if the guitar gets dry again it will be susceptible to cracking again, maybe in a different place, because you set it up for this problem on the day you made it. If you know it is going to live its future life in a high humidity climate you might get away with this, but it is a risk.
2. You could graft in a sliver of additional wood to effectively make the back a fraction wider, but without needing to deconstruct it. Do you have any offcuts from the back? If so you could cut a long thin splinter and graft it in and you would probably get a pretty good cosmetic result. If you want to do it this way I can give you some more tips about how to go about it. It would make it more stable in the future. The risk would be an imperfect cosmetic outcome, but it will be stronger than method 1.
3. Fix the underlying problem. Take the back off, remove the braces, repair the crack by humidifying, gluing and maybe filling the defect. You might need to rejoin the back with a centre strip (as you mentioned) to restore it to correct width. Rebrace it and then reglue it to the rims (and make sure you do these two steps at RH 40-50%). More work, but best possible outcome.

Whatever you do, get yourself at least two RH measurement devices and watch them carefully. If you want to get into controlling RH in your workshop there are lots of good threads at ANZLF to advise you.

JurgenV
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Re: Crack repair posssible?

Post by JurgenV » Sun Jul 17, 2022 6:19 pm

Thank you for the detailled answer.

Actually I do have a hygrometer in my basement which is the reason I know that the rel. humidity changes sometimes a lot. Ususlly I just glue cross grain if I´m around 45-50% rel. humidity. But the crack is prove that something went wrong this time. I would have thought the influence of the braces would be the main factor but you´re right I totally forgot about the sides as an additional factor.

I think option 1 and 2 are not what I want to do. That leaves me with option 3. Routing the back off, planing the braces off, repairing the crack and try to put it back together. For the crack repair: Would you suggest titebond or superglue or something totally different?

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Mark McLean
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Re: Crack repair posssible?

Post by Mark McLean » Sun Jul 17, 2022 8:42 pm

Maybe you did all the right things and this is just a crack-prone piece of wood? But it is very beautiful! I have never worked with Padua so I am not sure if it is more prone to cracking than other timbers. I think your plan sounds good. I would use Tightbond Original, maybe diluted a bit to get it to work right into the crack? If there is any surface defect you could then work in some sawdust with more TB or CA.

Dave M
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Re: Crack repair posssible?

Post by Dave M » Mon Jul 18, 2022 3:11 am

Not helping the original question but Mark, when I tried build with Padouk I found it would crack if you just looked at it sideways! Bending the sides was hopeless I just ended with cracks half way along the length. I gave up as I didn't want to send a box out into the world that was so liable to cracking.

Now again I might have been unlucky in the pieces I had...?

Cheers Dave
------------------
Dave

JurgenV
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Re: Crack repair posssible?

Post by JurgenV » Mon Jul 18, 2022 3:50 am

ok, actually you didn´t ask me but I think you just had a bad batch. I didn´t have any problems bending the sides. Didn´t even have to change my standard settings for temperature. And it´s the second guitar using Padouk. Just the other one didn´t have the sap wood stripe.

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lamanoditrento
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Re: Crack repair posssible?

Post by lamanoditrento » Mon Jul 18, 2022 8:49 am

I would be tempted to add marriage strips along each side of the sapwood if you are taking that back off. Clearly a weak point in that piece of wood.
Trent

JurgenV
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Re: Crack repair posssible?

Post by JurgenV » Mon Jul 25, 2022 5:03 am

Need some advice.
If I want to take some of the sap wood area away and fill in a new bigger piece of wood as a wedge : What would be your recommendation on how to do it (inserting the new piece without some "binding" at the interface) and what kind of wood?
At the moment I just have two ideas:
Either inserting a piece of Padouk (red) in the middle (so that I would still have part of the sap wood on either side (red sapwood red sapwood red ) or trying to use maple as the wedge (red sapwood Maple sapwood red)
I´m not sure if anyone understands what I´m writing here :) If not I can try to make a sketch

At the moment I´m not really sure on how to continue with this guitar disignwise. Any suggestions appreciated

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kiwigeo
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Re: Crack repair posssible?

Post by kiwigeo » Mon Jul 25, 2022 10:49 am

JurgenV wrote:
Mon Jul 25, 2022 5:03 am
Need some advice.
If I want to take some of the sap wood area away and fill in a new bigger piece of wood as a wedge : What would be your recommendation on how to do it (inserting the new piece without some "binding" at the interface) and what kind of wood?
At the moment I just have two ideas:
Either inserting a piece of Padouk (red) in the middle (so that I would still have part of the sap wood on either side (red sapwood red sapwood red ) or trying to use maple as the wedge (red sapwood Maple sapwood red)
I´m not sure if anyone understands what I´m writing here :) If not I can try to make a sketch

At the moment I´m not really sure on how to continue with this guitar disignwise. Any suggestions appreciated
My thoughts...the process is a bit like a bigger version of fitting a butt wedge. Inserting some purfling along outer edge of the wedge gives you a bit of "wiggle room" if the fit isn't perfect. Any gaps become more obvious if the wedge and back are similar in colour/texture.
Martin

JurgenV
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Re: Crack repair posssible?

Post by JurgenV » Sun Jun 11, 2023 3:44 pm

As there is no customer anymore for this one I had put this project on hold but finally coming back to it. It will probably go to my brother as he suddenly showed some interest of getting a guitar for himself.
I managed to repair the crack and to find a piece of padouk with the right colour to put between the original halves and this is what it looks like at the moment:
1686460913495.jpg
Actually I think it looks quite nice. At least not too bad :)

As I never attempted a similar thing: How do I reinforce the three glue lines (repaired crack and the two joint lines)? 3 back strips are perhaps a bit much. So how do you guys reinforce these potentially weak spots on this guitar back.

two backstrips for the inserted padouk strip are enough? or just for the crack line. or on all three? I don´t have a clue what makes most sense in this situation.

Hope to get some advice from you as there are a lot of very experienced luthiers in this forum
Last edited by JurgenV on Sun Jun 11, 2023 8:13 pm, edited 3 times in total.

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kiwigeo
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Re: Crack repair posssible?

Post by kiwigeo » Sun Jun 11, 2023 6:47 pm

I'd go with two back strips and some cleats across the crack. If the crack extends for some length then a third back strip
Martin

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Mark McLean
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Re: Crack repair posssible?

Post by Mark McLean » Mon Jun 12, 2023 9:36 am

That is looking good. Different, but interesting. It is hard to know the perfect answer to reinforcement, but I think Martin’s advice sounds good. Have you made the neck yet? If not, a 3 or 5 piece neck with the same longitudinal stripes could be good?

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Taffy Evans
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Re: Crack repair posssible?

Post by Taffy Evans » Mon Jun 12, 2023 2:33 pm

Hi, On my three-piece backs, and two-piece sometimes, I often use little spruce diamonds for the length of the join, with maybe 20mm or so between them. Just to be different and I think it looks classy. I can't find a photo of one, but here's a back I did similarly using heart-shaped reinforcing cleats.
3-DSCF8290.JPG
3-DSCF8290.JPG (90.96 KiB) Viewed 18697 times
I found it
9-SAM_0652.JPG
9-SAM_0652.JPG (119.63 KiB) Viewed 18696 times
Cheers Taff
Taff

JurgenV
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Re: Crack repair posssible?

Post by JurgenV » Sun Jul 02, 2023 7:28 pm

Perhaps that would have been the better solution. I went for two marriage strips and a few diamond patches. Almost forgot to take a picture so had to do it just before closing the box.
IMG_20230630_154533.jpg

JurgenV
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Re: Crack repair posssible?

Post by JurgenV » Sat Sep 30, 2023 8:04 pm

I still owe you the final result. I wasn´t sure if I should post it in the Gallery but here you can see the whole story.
IMG_20230930_112706.jpg
IMG_20230930_112854.jpg
IMG_20230930_112948.jpg
IMG_20230930_113019.jpg
As I didn´t have a customer for this guitar anymore I decided to add a feature I was alway interested about as soon as I got comments from electric guitar players like "can´t you go lower with the action" even when the action was already at 2.5mm.
=> an adjustable neck angle. I used an adaptation of Mike Doolin´s design which means I can change the neck angle through a hole in the heel. If you don´t know the details about Mikes design check on his homepage

http://www.doolinguitars.com/articles/adjnecksys/

Instead of using a tenon joint I used a butt bolt on neck and tried to reinforce the fret board similar to the bolt on bolt off neck of Trevor. First mistake was trying to hide the gap under the fretboard by using the full width of the fretboard. As I have a fixed screw in the heel I can´t just slide the fretboard in place from above but have to come from the front. Therefore I had to widen the opening and now the gaps in the top at the side of the fretboard extension is more visible than planned
IMG_20230930_112722.jpg
In addtion I had to adapt the pocket for the heel as I did a mistake when aligning body and heel so the marks were slightly off and again the gaps around the heel are bigger than I planned for.
IMG_20230930_112755.jpg
IMG_20230930_112802.jpg
IMG_20230930_112808.jpg
But other than that the guitar sounds quite nice. The overall appearance is a bit flashy (for my taste...)

The specs:
top: cedar
back and sides: padouk
binding: figured maple
bridge: padouk
neck: swietenia
fretboard: indian rosewood

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kiwigeo
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Re: Crack repair posssible?

Post by kiwigeo » Sun Oct 01, 2023 9:23 am

Nice work dealing with that crack.
Martin

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