Ahhh. My first returned project. This cello had a cracked neck a few weeks ago. I squirted some glue in the crack as best as I could. It seemed to hold up after several days at my house with full tension on the strings so I figured all was well and shipped it home with a student. To my dismay a week-ish later the crack had opened up again.
Here's what I learned:
NEEDS MORE GLUE!
In order to get more glue in the crack I had to use a slightly less comfortable method that involved essentially over-tightening the strings and then yanking on the neck a lot to open up the wound as much as possible while I squeezed all the glue I could get into the crack from every possible angle. I also used a different clamp this time that seems to be holding a bit better than the wood clamps I had used previously.
And of course, as I am writing this I remembered that I totally have syringes from the printer ink refill kit I bought (and can no longer use) that would be PERFECT for injecting glue in small cracks. I suppose if it comes back again I have another game plan to try out. Until then I get to play the waiting game (totally the worst part of this job!).