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Preexisting stable crack opened up when the humidity dropped. It closes fairly well (but not 100%) when I press down on the soundboard. This guitar has lots of wear so I'm not worried about the cosmetics per se (the crack showed before it opened up) but I would like to prevent it from getting any worse or extending up under the bridge. Tried taping across with doubled up masking tape but it just stretched so I'm probably using the wrong stuff. Any suggestions? Particularly with regard to what kind of glue to use? I figure epoxy would fill any gaps but I don't see how I can get it in there without having it smear all over the soundboard. Titebond would get in but I'm not sure I can guarantee enough pressure to get the edges to bond. I'm worried if I get a bunch of glue in there and it separates again then I'll have no other options. Before someone says "take it to someone who knows what they are doing" - yes, I know that but this is primarily for my own fun and education so doing that would defeat the purpose. Thanks in advance to anyone who can share some words of wisdom with a neophyte.

What kind of guitar is this?

I'd suggest you bave a look at Frank Ford's pages.
Try this one for a start: http://www.frets.com/FretsPages/Luthier ... crack.html
There are a lot of similar projects where he explains various methods.

It's possible the braces underneath have come loose, so you'll have to have a look.
First re-humidify the guitar; this often shuts the gap by itself.
Please, no epoxy. This would be a good place to use titebond.
You'll probably have to glue some cleats for this kind of crack if you tackle this yourself.

Also, make sure to control humidity so the crack stays stable and no other appears.

Also see post number 4 from Hesh right there:

viewtopic.php?f=10137&p=612797#p612797

Just what I was looking for. Thank you!!

Hesh here, the whack-job who wrote that post...

Or... you can take it to someone who knows what they are doing....

Seriously though those cracks look like they need a pretty good approach and very likely will not close and stay closed on their own without encouraging another crack somewhere else. What kind of guitar is this?

Also good thinking on not wanting to try anything until you are sure of the approach not wishing to contaminate the joint. I wish more folks considered that as you rightly did - good going!

Rehumidified the guitar but it took me several weeks to get back to it. Now the crack is very tight - no light gap or movement at all on either side. braces look secure. I'm not sure i could open it up enough to get any glue in there without excessive force and all that could go along with that. What's the right thing to do? Would you tell a customer just to keep the guitar from drying out again and leave it be? I guess I could put cleats in without gluing the actual crack but I'm not sure what purpose that would serve. Thanks.

OK here is one method to proceed and it's what we have done with hundreds and hundreds of cracked guitars so this is not shade tree Lutherie but commercial top quality practices.

If you are not going to fill the crack and want to have some risk of it cracking elsewhere and since it's closed now I assume that is your intended direction.

1) Wash your hands. Seriously! The wet glue that you are going to be rubbing into the crack will pick up any dirt in your fingers and let it migrate into the crack making the crack show a dark line forever more....

2) Use Titebond original for a couple of reasons such as a) it works great.... and b) you are new to this and likely unlikely to be equipped for HHG either experience wise or even having any.

3) With clean hands and arm in the sound hole and depending on your specific arm this can be difficult to reach but try anyway - reach inside the box, no strings of course, and pump the crack up and down from the underside. This can seem impossible to do but stay with it and you will eventually see glue migrating to the underside of the top.

At the same time lay a bead of Titebond original (you could use extend too) on the crack top side and with that clean finger while pumping the crack... rub glue into the crack. The pumping action will help the glue migrate down into the crack.... on the guitar....

4) Have a small mirror and flashlight and while looking inside you will want to see little, tiny glue beads on the underside of the crack verifying that you have glue in the crack.

5) Here is some more stuff that you will need and it will be a good idea to do a dry run with the clamping system that you use:

a) A flexible but semi-stiff caul of sorts, I like plexiglass so I can see though it longer than the crack and 2" or so wide.
b) Waxed paper
c) A wet paper towel with clean water and maybe a cup of clean water standing by
d) Long reach clamps what will reach to the bottom 1/3rd of the crack and another one that will reach about 2 - 3" less. If using clamps I would use two.

Or rare earth magnets, the special ones that are 3/4" diameter and exert at least 40lbs of clamping force.

6) Once you have glue in your crack... sorry, I really am like this.... wipe the top of the guitar where the glue was being rubbed in with the wet paper towel. You want it damp, not dripping so that no more moisture is introduced into the crack. Clean up the topside of the crack, lay down the waxed paper, lay the clear plexi caul on that and either install the clamps or the magnets. The goal here with this step is to have both sides of the crack forced level with each other as the glue dries.

Be mindful of what the clamps are bearing on inside the box too so that you don't glue them, or the magnets in place, waxed paper is your friend when it comes to glue.

Hang the sucker and walk away for 24 hours.

The next day:

7) Make small spruce cleats that are diamond shaped with all four edges beveled to nothing. The grain should be running at 90 degrees to the top grain direction and the cleats are installed over the crack like a band aid.

For a crack that length I would use at least 3 cleats, possible more and you want them located between braces. It's a good idea too to see if the braces are at all loose where it cracked and if they are push glue under them with a feeler gauge or we use hypodermic needles in various sizes and reclamp again with a protective caul tops side of the instrument.

Cleat sizes for this one and again this is how I would do it would be 5/8" X 1/2" X .050".

8) Cleats can be rub jointed with Titebond or I use small rare earth magnets that only pull about 15lbs of clamping pressure.

If done well it will never crack there again but may crack, as mentioned somewhere else if exposed to low RH in the future and if dimensional instability is an issue with this instruments materials.

Good luck and if you want to PM me with questions I am always happy to help! Kudos to you too for trying this yourself!

Fantastic explanation as always. Thank you. At the risk of alienating a community of guitar repair experts, I was wondering if anyone would be willing to offer an opinion on the following: What is the natural history of these kind of cracks in the life of the instrument and can they be left unrepaired? Is there any particular or predictable effect on tone? I am far from an acoustical engineer but I can speculate that the soundboard might vibrate differently or that somehow impulses that were traveling across the lower bout would bridge the unglued gap less efficiently. I have to say that I have not noticed anything different in the sound of this particular instrument with the crack open or closed. What I'm really wondering is whether it might act like a kind of humidity safety valve. Obviously this top has demonstrated a weak point and how it wants to split under stress. If I glue and cleat it, and it dries out again (hypothetically of course), might it not just split somewhere else? Obviously there are cosmetic reasons why one would want to have the instrument repaired but from a purely functional point of view perhaps it would be OK to leave it. On the other hand, if the defects generally extend themselves out to the binding or under the bridge over time then obviously this could have more serious consequences and should be addressed sooner rather than later.

Contrary to what might be the belief of many cracks can be left open even for years and often are too. Good questions by the way! Will it/does it change the sound of a guitar, probably but.... this need not always be a bad thing either....

One of the down sides of leaving a crack open is that the braces spanning the crack inside the box can start to come loose in the crack location as well and loose braces can be heard and are not a pleasant sound.... unless maybe one is another loose brace....

Regarding the RH valve idea an open crack will keep moving opening up further and closing with RH changes so it does act like a valve of sorts but again it's also working the braces under the crack over and over too helping them to come loose in time.

Now since you are asking about the possibility of doing nothing I hope that my method for fixing it did not scare you out of trying the fix. Very likely some of the stuff that we use you may not have and that's understandable too.

As another option and we won't accept shipped in work so this is NOT a shameless pander of sorts this is not an expensive repair or should not be if you are not looking for cosmetic perfection. A crack like this should be able to be professionally glued and cleated with a professional set-up thrown in for just north of $120 at a quality shop. YYMV in your location of course.

There is also the issue of the crack being right smack in the center of some of the most valuable real estate on a guitar in terms of not only tone and projection but also this area needs to provide a stable platform for the bridge too. Permitting the crack to continue to move with RH swings may also lead to the bridge lifting in time. It's par for the couse for us to see these cracks WITH a lifted bridge.

With all of this in mind I would fix it or get it fixed.

Not scared at all - excited in fact. Just trying to understand the fundamental principles at work. As an aspiring amateur its always comforting to know that there are professionals out there who can save the day if the need arises. Thanks again. I'll let you know how it goes.

Hesh,
Sound advise on the Titebond. I previously tried CA, and never got as good of a close as using TB, also I have had CA react and had it mess up the finish on an old Kalamazoo.. I use Titebond all the time for this now. Thanks.
GS

For really tight cracks that are hard to get glue in I cut Titebond 50% with water and after washing my hands.... rub the thin, cut TB into the crack followed by a bead of full strength/viscosity TB. My observation was that the thick stuff would be drawn into the crack better is the crack was "sized" with thin stuff first. It's been working very well for me and this is the second crack season that I am using this technique.

I watched a Dan E./Stew-Mac video yesterday where Dan is describing some of the tools that Sm offers for crack fixes. Turns out and Dan says this too that the sizing the crack method as described above is how Martin at the f*ctory repair shop glues cracks. Go figure....

And I thought that I was on to something.... Just goes to show ya that there have been many, many before us and you know what? They just may have known a thing or two too!