When I fill body tube seams, I use carpenters wood filler (CWF) mixed with water to a consistency of mayo. Never have had any problems. I apply the filler with my fingertip to minimize the excess that will later need to be sanded flush.
If you do get some CWF where it doesn't belong, it's a pretty simple matter of sanding it off. Just apply light pressure with a fingertip to the spot.
One thing to bear in mind is that unless you're building a museum model, you are going to put a black powder motor in this rocket, boost it into the air a couple of hundred feet if the motor doesn't cato, hope the chute deploys properly and that the rocket avoids hazards such as trees, cars, roofs, and rocks on the way down.
Don't invest more into this bird than you are willing to lose.
From 15' they all look about the same.
Kit (AKA Cranky Kong)
Total Total Impulse as BAR: 7,753.69 Ns (Equivalent to a 51% M motor.)
=| Calirado, Colofornia...what's the diff anymore? |=