Hi Peter: It is suspect that the center seam below the bridge came open that is the single most common place for RH deficit cracks to happen. Here's what I would do but again I would be concerned that this guitar shows signs of perhaps being dry. 40% is on the low side and if it was built in say 55% that will be enough of a difference to make it RH sensitive going forward. How old is the guitar?
1) Get a car wash sponge and a large garbage bag (not an editorial comment on your building
).
2) Completely wet the sponge so it's soaking wet and place it in the bag being careful to not get the sides of the bag wet.
3) Place guitar in bag and tie the bag snuggly where the neck meets the body.
4) Hang guitar by the headstock on a hanger with wet sponge in the bag below the guitar and the bag closed at the neck joint. Leave it for 1 - 3 days checking daily for the crack to completely close. You don't want the sponge or water to ever touch the guitar. Our measurements show inside our bags to be about 75% RH if you are interested.
5). When the crack is closed remove the guitar from the bag and wash your hands. We wash our hands because Titebond original what I would use will carry dirt from your fingers into the crack and then the crack will appear darker. So wash the hands please. You don't need to attempt to clean up prior Titebond regardless of what anyone may say.
6). With Titebond I use two steps and this is my own method proven on hundreds of cracks that I've done.... wait....
I mix a little bit of Titebond Original with water, 50/50 and with one arm in the guitar I run a bead along the crack top side. The guitar is level on my bench. With the inside hand I pump the crack with my fingers and the capillary action sucks that bead of thinned Titebond into the crack nicely. When I feel wetness from the glue on my fingers inside the box all along the crack step one of the gluing is complete. Now immediately run a bead of full strength Titebond Original along the crack top side. Repeat the pumping of the crack with your fingers inside the box. You are trying to get the glue topside to migrate into a tight crack. I work the glue topside into the crack as I pump inside the box. the pumping from inside the box should be forceful enough that you can see the crack slightly open and close as you press and release.
Again when you feel little beads of the thicker glue on the inside of the top you have the glue well distributed in the crack. This method exploits watered down glue to encourage the thicker glue to follow into the crack and it works very well.
Once the crack has glue in it and will stay closed on its own I wipe excess glue inside and outside off and slap down waxed paper topside over the crack. You can use thin plexiglass, cutting board material, lots of things but I like it clear over the waxed paper as a caul that once clamped will level both sides of the crack to each other. Any clamps that go inside should have waxed paper on the business end or any cauls Titebond will not stick to waxed paper.
I use rare earth magnets and not the whimpy StewMac ones we buy ones that exert about 42lbs of pressure and can be a bit dangerous since they can jump. Or you can just use clamps, long reach clamps with the flat cauls to keep both sides level. One of the nice things about magnets is you can position them topside for where you need them inside where you can't reach.
I leave it clamped and glued over night and then remove clamps. Hot water cleans up excess glue. If you level well there is NO finish touch up necessary and since this is a center seam if you washed your hands the repair may be invisible and level.
But I remain concerned that it did happen when our furnaces have started to come on and this fall in Michigan has been exceptionally dry too....
Lastly I make little diamonds perhaps 1/2" long beveling all edges and glue them inside the box cross grain between any tone bars and such. These are cleats and insurance. Cleats can be Titebond too but there is an argument for a drop or two of medium CA as well. A cleat does not need to be serviceable.....
And really lastly it would not surprise me if the RH is not lifted where this guitar lives if it cracks again since this does sound like an RH crack to me.
Let me know if you need any help and PM me I am always happy to help.
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