It's nitrocellulose, and is very flammable. Think "gunpowder". Dried pieces burn instantly! Be careful with flame when putting points on a finished shaft, or you may be starting over!
If it gets too thick, it can be thinned with acetone, also extremely flammable. Pour some in the tube, and pump an old shaft in and out to mix it. If it cracks in use, it was probably too thick when put on. I have some that's been in the dip tube for more than 10 years, and I periodically add more and thin it with acetone to renew it. It should have a consistency much like room temperature honey. If too much wipes off on the gasket, it is a little too thin. A little thin is better than a little thick, IMO, because of the cracking potential. If it's a little thin, you just dip it a couple of more times until the coverage is good.
When I use polyurethane, I use a penetrating sealer first followed by the gloss finish straight from the can, and brush both on. I quit using poly when I was shooting 3D a lot- poly sticks badly in 3D targets. GL doesn't.
Superglue works great for fletching with poly, but dries more slowly with GL. Duco is better for the GL.