I'm curious too, so I will look at it again tomorrow. (It is pouring rain right now, and supposed to keep coming down---so no flying for a few days. Everyone knows it doesn't rain in California--except on the weekend.)
The stuff was clear, and "crispy hard" like I have seen Ambroid and Sigment get when I used them before. When I was cleaning up globs of glue in a corner, I hit it with Tim's favorite tool---the Dremel "death wheel" to get a square corner back between the engine bearers and the bulkhead. That caused a noticeable smell, so I want to go hit a scrap and see if I can tell what it is. I was kind of focused on the job before, not the autopsy....
Soaking the joint and inevitably the balsa on both sides of it did not begin to soften it up. So I understand everyone's input that either the acetone was evaporating too fast, or it isn't a cellulose glue.
Today I built a completely new stab and elevator set with a decent wire joiner instead of a wood stick with a scrap of carbon on it. So I'm good to go. The incidence is gone, and the fuselage joint surfaces cleaned up great. Going back together will be no problem. (If you are going to work on an OPP, it is MUCH nicer when it is not already painted, and even better that it has zero oil soaking.)
My guess is that this was built (actually, partially built) about 12 years ago. Whatever kind of glue it is, has had a lot of time to get hard.
The whole glue thing is interesting when you are doing rebuilds of OPPs. Not to mention the strange stuff I've seen on ARFs. Some of it looks like UV-cured glop, which sure would speed thing up....
Divot McSlow
PS--Now the plywood bellcrank is a whole 'nuther story....