Most of the time, the guitars I restore are taken apart as part of a GammelGura restoration. But sometimes I have to work through the sound hole during regular repairs, the problem is usually that the braces inside are cracked or loosened at the ends. The solution is simple, insert glue and clamp the brace against the top or bottom until the glue hardens. Simple, but not really.
Getting the glue under the brace or in the crack through the sound hole usually works well with a small metal spatula. These can be bought, among other things, from Stewmac in the USA, which has most of what you need for guitar repairs. The most difficult part is gluing the braces deepest inside the guitar through the sound hole. You can use long clamps or small wooden sticks of exactly the right length between the brace and the cover or bottom, but it is usually difficult to get everything in place, and you have to make new sticks that fit the guitar you are repairing. Another method is to use super magnets, but then you have to use powerful magnets that sometimes unexpectedly find their own place in existence... Both methods work, but I wanted to find another and simpler way.
I got the opportunity to test a so-called air wedge in a small Levin where all the three bottom braces were loose at both ends. The wedge is made to, among other things, adjust doors in height during installation. It is a small, airtight and strong bladder with a rubber pump that you can use to inflate the bladder. Without air, it is thin and flexible and can easily be inserted through the sound hole. The one I bought on Amazon is 11 x 18 cm. When inflated, the bladder is about 6 cm thick and according to the advertisement it should be able to lift 200 kg!
The air wedge was completed with an aluminum caul in the form of a U profile and a small adjustable foot to be able to push to the end of the brace, the part that is most important to glue. To give the bladder a large surface to push against, I made a block of wood with a trench where the aluminum profile can be fitted. The block can be made with different thicknesses for guitars with a low or high side, you want to get the distance to the opposite top or bottom less than 6 cm.
The adjustable foot is adjusted to the brace that is most easily accessible through the sound hole, all braces in a guitar usually have the same slope at the ends. Through the holes in the U profile, you can see that it has contact with the top of the brace at the same time that the adjustable foot has contact with the end. Once the foot is adjusted, a nut is used to lock the foot in place. The wooden block can then be clamped onto the U-profile.
The most difficult bottom brace is the one furthest into the guitar, but it's easy to feel your way and thread the caul over the brace in the middle and then slide it into the correct position at the end of the brace. With the caul in place, I simply place the air wedge on top of the wooden block and inflate the bladder via the rubber bladder and tubing long enough to reach out through the soundhole.
But before you pump up and put up to 200 kg on the caul, you must first mount two cauls with clamps on the outside of the guitar to counteract the force from the air wedge, otherwise the guitar will be crushed from the inside!
An advantage of the wedge is that you never have to think about what sits opposite the rib you need to glue. The bladder forms against any opposing ribs in the lid or bottom. I also have pieces of U profile in different lengths without feet at the end if a brace only needs to be glued in the middle or if the brace is short or lacks thinning at the end.
All six gluings in the little Levin went quickly and well. To test it out, I inflated the bladder as much as I could (it has a safety valve in the pump itself) without any problems. I used a third clamp from the outside, right where the wedge was placed inside for safety. It is enough to pump it up until you can see that glue is pushing out from under the brace and that the joint is tight. Which is recommended.
I will use this jig in the future and avoid having to custom-make wooden cauls and work blindly.