Applying poly glue-a different opinion

[Gray E Fowler]

When using the current polyurethane glues for skins I think there are a 
couple important things to consider. From what I have read on the list, 
there are several things being offered up that I kind of disagree with and 
I will say why.  First, the THIS polyurethane does require moisture to 
cure, but remember-gaseous water , that is humidity.  I am not so sure 
where the threshold is but I am sure that if you are about 40% RH you 
should be fine. Spraying/wiping  the wings with water is really excessive 
as most of that will never evaporate in time to do any good, and what you 
will have is a wet wing. I would suggest that if you gotta do it, do the 
foam, be sparing, as the balsa has plenty of moisture in it...unless you 
live in the western US (or the outback -Tom) where RH is very low ...like 
10-15%. 
Second, I have alway disliked the hair spray idea because the bond between 
adhesive and balsa can now be seriously degraded by humidity. Remember 
that hairspray is water soluble, cuz if not one application would do for 
life.....without getting into too much detail...humidity degradtion of 
bond joints is serious, and now using hairspray just creates a real weak 
link over time.
Also-the hairspray idea I think originated when most were using epoxy, and 
it would soak into the balsa requiring excessive amounts to adquately bond 
the skins. With this urethane that expands, it fills the bond line very 
well. and the viscosity increase is so great that alot is not going to 
soak into the wood, AND you want that anyway for a better bond. Adding HS 
will just give you a weaker bond, with the added weight of the HS itself.



Gray Fowler
Principal Chemical Engineer
Composites Engineering
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