Hi Wayne,

I had a debate with a buddy of mine about moisture in vacuum bags. Here's my experience.

I vacuum bagged a wing with contest balsa, PolyU and misting (perhaps it was too much?). Only the wing was in the bag. The bag was placed in the cradles on a flat table, and I used sandbags to weight it down to keep it flat. After about 15-20 hrs, I removed them from the bag and was pleased with how straight they were. I also noticed that there was dampness on the balsa skins. About 15 minutes later, they started to dry out and warp.

I would think that when not in a bag, the ambient air would absorb the moisture out of the balsa much better. Anyway, I'm still bagging with PolyU but not misting, and even in the dry winter here in PA, I have not had any problems with cure. As to penetration, after cutting out a cross section of the wing, I was surprised to see that the PolyU penetrated the foam as much as 1/4".

I agree about PolyU being much easier to work with than epoxy. And it seems lighter as well.