MIMF

the center seam of my guitar, between the bridge and rosette, has parted slightly (left it for 6 mo in hot, dry environment). I used SM instrument glue when I built this guitar. There's a center brace under this crack, so magnets won't work for clamping. So my plan is to fashion a piece of wood on the inside so I can get clamps in place. I am thinking that HHG would be the right thing to use, but am wondering if there would be a problem caused by the instrument grade glue in the original joint. Any and all advice will be muchly appreciated

You will need to re-humidify the guitar to close up the seam so it can be glued, but first you need to clean out the old glue. Care must be taken to avoid staining the wood with any solution you use. White vinegar will soften and ease the removal of aliphatic glue (Titebond, LMI White Luthier's Glue, etc.) but keep any ferrous metal away from it or you will cause dark to black stains in the wood. Use plastic or glass implements, like a c heap stiff artist's blush with no metal. The area must be rinsed after the cleaning to remove any of the solution, and thoroughly dried before humidifying, and then gluing.

I use a plastic trash bag or similar to put the instrument in, a damp ( not dripping!) paper towel inside the sound hole, and close it up in the bag for a couple days. Inspect it and if the seam is closed proceed with the gluing process, if not closed use another paper towel and close it up for another couple days. Any seam or crack that need the re-humidification process should be cleated on the inside or otherwise reinforced or it may open again when it dries out.

It is imperative this process is NOT RUSHED. Wait for the seam to close with humidity, and DON'T put more than two paper towels in the guitar at one time unless YOU are taking responsibility when it goes wrong.

Michael: Thank you for that info. I knew I had to rehumidify the instrument, so put some damp sponges enclosed in slotted sandwich bags inside the sound hole. Unfortunately, I did that before I got your input.
The humidity inside the box is at about 43% right now. Should I let it dry out and then proceed? What's your input?

Put it in a large plastic bag as Michael explained. You need to get moisture into the guitar. What you have done so far is not wrong, just not effective.

Okay. Soundls like I should clean the old glue out first. Then re-hydrate the box. I'm including a picture.

I'm probably over worrying this but here goes. This is a hairline crack. I don't think I will be able to get any brush bristles into the crack. I can do the vinigar trick and just flush the old glue out, I guess. Anything else you gents can tell me? Sorry to be such a pest.

Sometimes it helps to use unwaxed dental floss if you can get into the crack. Soften with the vinegar and then the floss.

Thanks Mark. There's a brace that runs most of the way under this crack. So I can only see about 1/4" of the crack when I use a light and mirror. I could get the floss into that section, thus also vinigar and glue. If I try the vinegar approach the whole length of the crack, I might cause the brace to loosen up. I'll try the floss idea on the part of the crack I can see. I don't think there's much I can do other than start cutting away and glue in a sliver.

Be careful not to make the situation worse. You don't want to destroy the joint by trying to clean it out. A brace underneath it will prevent most access. I would be tempted to work some glue into the joint and let her rip.

The best way to work glue into a tight joint is thin the glue with about 5 to 10% added water to Titebond. Now lay a bead on top of the joint. Put one hand inside the guitar and press your thumb under the joint. This tends to open the joint on the outside. Now use your free hand to massage the glue down into the joint. Once you release the inside pressure, you should see a tiny bit of glue squeeze out of the joint. If so, you know that glue penetrated the joint. Now use a slightly damp rag to remove excess glue on the surface. Clamps may be added but if the joint is tight it may not be necessary.

If the seam is blocked by a brace underneath then Barry's suggestions are the best way.

Another way to force glue into a crack is to lay the thinned glue in a small bead over the crack, and place masking tape over the bead. Seal the edges of the tape and then you can push the glue into the joint. Remove the tape, and wipe off the excess.

What Mike Lewis said.
I'd try a little bleach-- start with hydrogen peroxide. If that doesn't work,
try Oxalic acid, and then chlorine bleach--easy does it, let it dry between treatments.
A cheap artist brush can be trimmed to make the hair stiff enough to scrub.
If using Oxalic acid or bleach, I seem to recall, you should neutralize the ph.

If you've used vinegar to remove the glue, keep in mind that the middle brace may have been loosened as well.

Finally an idea I've never tried, but which may work: remove that center brace if you can. If it's not broken put it back, or cut and mill a replacement. But, before replacing it, steam it or keep it or otherwise raise moisture content of the wood to swell it, and attach it after the top has swelled back and closed the split. You can also cleat the crack and notch the brace. I prefer diagonal grain orientation to cleats that run perpendicular to the top grain; again the idea is to get the cleat to shrink in a way that will pull the to halves together.

Filling the crack with a splint is absolutely the last option. It's virtually impossible to make an invisible, stable repair.