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Suggestions on closing a back crack in maple?
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Author:  Ken Jones [ Fri May 01, 2015 1:24 pm ]
Post subject:  Suggestions on closing a back crack in maple?
I have a Euro maple guitar with a fairly wide crack right at the edge of the center strip, about 3" long. It's curious, as the center strip is 1/4" wide on the inside of the guitar, but only appears as a pinstripe on the outside, so it must be rabbeted into the maple. There's no graft on the inside. Here's the outside:
Image

And the inside:
Image

Easy to stabilize and cleat, but the repair needs to be as clean as possible, and I know from experience glue lines are very difficult to conceal in maple. I'm considering trying to humidify it in a trash bag to try and close it, but am wary of possibly causing other issues in the process. Any suggestions?


Ken Jones
Mountain Song Guitars
Author:  Greg B [ Fri May 01, 2015 2:55 pm ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Hot hide glue is the best fix for this by far IMO. Done well, it will be invisible. You'll want to humidify to close the crack.

That's what happens when no back reinforcement strip is used. The joint doesn't fail, but you get a crack right next to it. I found this out the hard way the one time I didn't use one. Is the weird insert line thing flush with the back? I'd add a full strip if possible, or at least one that covers the joint over the entire lower bout.
Author:  Hesh [ Fri May 01, 2015 3:15 pm ]
Post subject:  Re: Suggestions on closing a back crack in maple?
"but am wary of possibly causing other issues in the process. Any suggestions?"

No need to be concerned about bagging em and tagging em. In the last several months we have bagged hundreds of cracked instruments with no issues resulting from the rehumidification process. We use a large, clear garbage bags... with a large car wash type sponge in the bottom and around 8 ounces of water either in the sponge or the bottom of the bag.

Strings should be off of course and the bag is tied snuggly around the neck above the body joint. Our measurments told us that the RH in the bag will be around 75%.

For badly dried out instruments we might keep them in the bag for four days, most for three. We have bagged longer and no issues either but I would not recommend it.

Now here is another issue.... So we rehumidify and force the crack closed but what happens if we left the instrument in normal RH for three more days. If the crack opens again, and it likely will this is indicitive of additional stresses that have to be considered.

RH related dimentional instability can be permanent in the sense that we can force a crack closed but the back wood may now be forever narrower....

This is why very well closed, cleated and glued cracks can at times result in more cracks elsewhere because of the dimensional instability and forgive me please.... shrinkage... of the back that's now for keeps. The more seasoned the back wood was to begin with the less this is going to be an issue.

In situations like these it may not be prudent to force a crack shut and glue and cleat (unless you don't like who the guitar belongs too (Just Kidding...)) because it can result in more cracking.

But I digress. I would be interested to see how it closes over three bagged days and what happens in 45% RH three days after that. It may be that the forced closed crack cleated and glued may work fine. It also may be that filling is a better structural approach but now we have figured wood to consder, matching, etc. as well.

Don't ya just love repair work.....
Author:  Ken Franklin [ Sat May 02, 2015 12:20 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
I think you'd have better luck routing out the ebony center strip next to the crack. Then glue the crack with hot hide glue and force it together with some temporary wedges. Then you can rout out the whole center strip and put in a wider center strip. When you're done you can add a wide reinforcement strip on the inside. This way you won't have to find out the hard way that the guitar wasn't built with well seasoned wood in a properly humidity controlled environment.
Author:  Ken Jones [ Sat May 02, 2015 7:25 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Hmmm, taking a router to a $10k guitar for a small crack repair hasn't occurred to me...

I think I'll try the trash bag approach...

Thanks guys!


Ken Jones
Mountain Song Guitars
Author:  Hesh [ Sat May 02, 2015 7:45 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
You will likely be OK just rehumidifing, gluing and cleating sistering the cleats to what's proud of the back plates where the BJR (back joint reinforcement) would normally be. By the way this may be a good example of why again the traditional BJR with some width to it is a good idea.

The more repairs I do the more I realize that traditional construction methods for guitars are actually pretty good in lots of respects.

Anyway once closed, glued and cleated this instrument will likely be more problematic with RH swings in the future. When we do these kinds of repairs, and that's nearly every day lately, we make sure that the current steward goes home with the repaired guitar AND a decent case humidifier as well as a brief educational discussion. We like to tell folks that at least in our part of the country if your furnace is turning on it's time to either properly humidify where the guitars live or use a case humidifier and be sure to charge the thing too. We have been known to give humidifiers away to folks too who may be short on funds.

By the way and this is just a general statement put out here and not directed at anyone specifically when I was building and selling all of mine were delivered with a decent guitar humidifier AND instructions as well as that discussion. In the event of abuse where the current steward let the thing dry out you don't want folks considering cracks warranty claims (unless for certain kinds of cracks) and supplying a decent guitar humidifier is a very small price to pay to both drive the point for humidification home and shift responsibility/culpability to who's got the thing now.

You would be very surprised just how few folks realize that purchasing a nice acoustic guitar also is a commitment to care for the thing including some often inconvenient tasks such as schlepping water all winter long....
Author:  David Wren [ Sat May 02, 2015 8:38 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Couldn't agree more with everything Hesh says. Too often there are discussions about humidifying guitars to close cracks and no discussion about letting the instrument come to some kind of dimensional equilibrium before gluing to see if the crack opens at all. Cudos!
Author:  kencierp [ Sat May 02, 2015 9:49 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Goes without saying -- +1 for bagging and vigilant proper humidification is a must. I would not get within ten feet of that guitar with a router, but honestly don't understand the suggested routing fix.
Author:  Hesh [ Sat May 02, 2015 10:08 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Yeah there are some differences between building and repair.

To use golf as an analogy... with building we hope to get to eventually use and enjoy every single club in our bag...

With repair sometimes simply repurposing a golf tee will get the job done.....

Ask K&K.... :)
Author:  Ken Jones [ Sat May 02, 2015 10:38 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Indeed, Hesh, that's where I get my golf tees!


Ken Jones
Mountain Song Guitars
Author:  Ken Franklin [ Sun May 03, 2015 2:29 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
kencierp wrote:
Goes without saying -- +1 for bagging and vigilant proper humidification is a must. I would not get within ten feet of that guitar with a router, but honestly don't understand the suggested routing fix.


The routing fix is in response to Hesh's observations that after closing the crack by bagging, a crack can reopen or create other cracks elsewhere. In my experience if something happened once it will usually happen again, maybe not right away but eventually. From the pictures, that doesn't look like a small crack to me. If this is a $10,000 guitar I would be surprised if the owner doesn't know something about humidity and all the more reason to do your best to keep it from happening again.
Author:  kencierp [ Sun May 03, 2015 8:35 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Ken --- I understand your disagreement with the bagging process and moisturizing care after the fix. I am just not visualizing the major surgeory you proposed as it applies to this completed guitar.
Author:  Clay S. [ Sun May 03, 2015 8:55 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
I would consider how old the guitar is and when the crack occurred. If it was recently constructed there might be a problem with the wood. If the guitar has been around for awhile and the crack recently appeared, it may have happened because of some extreme drying event. In that case I would humidify it, glue it , cleat it, and hope for the best.
Author:  Ken Jones [ Sun May 03, 2015 9:50 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
I think the problem is structurally inherent in the way the center strip is rabbeted into the plates -- the maple must be very thin over the 1/4" wide section, creating a stress riser at the inside corner of the rabbet, exactly where it cracked. It's humidifying as we speak, and after less than 24 hrs, the crack is closing. I think a back graft is definitely in order.


Ken Jones
Mountain Song Guitars
Author:  kencierp [ Sun May 03, 2015 11:30 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Ken Jones wrote:
I think the problem is structurally inherent in the way the center strip is rabbeted into the plates -- the maple must be very thin over the 1/4" wide section, creating a stress riser at the inside corner of the rabbet, exactly where it cracked. It's humidifying as we speak, and after less than 24 hrs, the crack is closing. I think a back graft is definitely in order.


Ken Jones
Mountain Song Guitars


Makes sense to me, is adding full length cross grain reinforcement out of the question? I know Rick Davis "Running Dog Guitars" uses really hefty material -- looks great!
Author:  Nils [ Sun May 03, 2015 1:41 pm ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Ken Franklin wrote:
I think you'd have better luck routing out the ebony center strip next to the crack. Then glue the crack with hot hide glue and force it together with some temporary wedges. Then you can rout out the whole center strip and put in a wider center strip. When you're done you can add a wide reinforcement strip on the inside. This way you won't have to find out the hard way that the guitar wasn't built with well seasoned wood in a properly humidity controlled environment.



Easier said than done...I personally would not consider doing that.

Basically I would humidify this until the crack closes and get some titebond in there. I'd also use a plexiglass caul to make sure it lines up right.
Author:  Hesh [ Sun May 03, 2015 2:32 pm ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Ken I was just looking at the pics of the inside again trying to determine if that center piece is flush with the back plates on the inside or proud of it. If it's flush simply installing a conventional back joint reinforcement strip over it and wide enough to bite into the sides would not only be a great cleat but further reinforce the entire back of the instrument. Pre-shaping the back joint reinforcement to bevel the edges to prevent another stress riser would be prudent too.

Clamping the strip could be done with magnets, carefully..., and shop made cauls with recesses for the magnets to keep them spaced and perhaps even a very slight positive radius on the bottom of the caul that would flatten out when the magnets are fully engaged AND spread the clamping force over the entire back joint reinforcement. Of course the thickness of the caul would be such too that the force of the magnets would over power that slight radius. Some teflon would be great for this and no waxed paper required either to prevent the caul from being a new "tone" caul and now part of the guitar.... :)

If the back strip is proud of the sides on the inside routing out the back joint reinforcement to be a good, snug fit to the strip would also really beef up this area.

+1 on plexi for the outside cauls which the primary purpose of is to clamp the two sides of the crack level as well as protect the outside of the instrument from damage.

I agree with you that the back strip is a contributor with the stress riser to the mishap and likely drying out as well and/or wood that was not well seasoned. In any event a perfect storm so-to-speak where various factors combined to crack this one.
Author:  Ken Jones [ Sun May 03, 2015 7:40 pm ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Yep, the center strip is flush to the back plates, so a graft will be easy.


Ken Jones
Mountain Song Guitars
Author:  Ken Jones [ Mon May 04, 2015 8:17 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
After almost two days "in the bag", the crack seems to be closing up nicely:
Image


Ken Jones
Mountain Song Guitars
Author:  Ken Jones [ Mon May 04, 2015 8:18 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Anyone else notice there's three Kens on this thread...?


Ken Jones
Mountain Song Guitars
Author:  SteveSmith [ Mon May 04, 2015 10:12 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
Ken Jones wrote:
Anyone else notice there's three Kens on this thread...?


Ken Jones
Mountain Song Guitars


Ha, now that you mention it. Being a Steve, I'm kinda used to that :P
Author:  mcgr40 [ Mon May 04, 2015 10:20 am ]
Post subject:  Re: Suggestions on closing a back crack in maple?
I can't quite grasp why it was built in that way. Was it just so with no reinforcement it would look prettier inside? Rabbeted in the maple has to make it weaker and a guitar is just sort of a giant cross-grain construction. I don't see it being easier to make than routing a strip in after joining the maple. After you reinforce it you will have moved the stress over to the full thickness maple, so I supppose it will be as strong as any back join then.
Author:  Hesh [ Mon May 04, 2015 12:32 pm ]
Post subject:  Re: Suggestions on closing a back crack in maple?
mcgr40 wrote:
I can't quite grasp why it was built in that way. Was it just so with no reinforcement it would look prettier inside? Rabbeted in the maple has to make it weaker and a guitar is just sort of a giant cross-grain construction. I don't see it being easier to make than routing a strip in after joining the maple. After you reinforce it you will have moved the stress over to the full thickness maple, so I supppose it will be as strong as any back join then.


Mike I can't know why the builder did this either but it does increase gluing area for the joint and has that going for it. Not using a conventional back joint reinforcement though likely contributed to the crack happening where it did.

You know using a decorative back strip and simply butting two back halves to it is also a way to get by when you can't shoot a joint well enough. I'm not at all talking about this guitar either. Many decorative back strips have fiber elements and will compress to help avoid any gaps but this is not something that I would be excited to ever do.

There are some f*ctory guitars that don't use back joint reinforcements either and if anyone is wondering if we ever see the back plates comming apart at the joint we most certainly do. Mind you it's always a result of drying out and abuse but this is where they seem to fail without a BJR, right down the center seam...
Author:  mcgr40 [ Mon May 04, 2015 3:28 pm ]
Post subject:  Re: Suggestions on closing a back crack in maple?
I thought the way to make the joint was to join the halves of wood, then put the strip in a routed channel about 40 percent of the way thru the back thickness, then put the reinforcing strip over on the inside. I glued my reinforcement on in the radius dish,not sure that really matters, but seemed like a good idea to me.
Making that rabbet in a piece .140 or so thick seems like extra trouble with no benefit(that I can see). I figured there must be some notion behind it.
Author:  Hesh [ Mon May 04, 2015 3:39 pm ]
Post subject:  Re: Suggestions on closing a back crack in maple?
mcgr40 wrote:
I thought the way to make the joint was to join the halves of wood, then put the strip in a routed channel about 40 percent of the way thru the back thickness, then put the reinforcing strip over on the inside. I glued my reinforcement on in the radius dish,not sure that really matters, but seemed like a good idea to me.
Making that rabbet in a piece .140 or so thick seems like extra trouble with no benefit(that I can see). I figured there must be some notion behind it.


Yep that's what I do, or did and it worked well for me too.
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