Applying poly glue-a different opinion

[wgalligan]

By lightly sanding the wood after an application of hairspray you would
be exposing the higher wood grain to the glue. Thus creating enough
bonding surface for the glue to adhere to.  
 
So far no problems as proven by you this weekend with your 45 degree
climbing 80mph full power on Mintor 1.70 outside snap that didn’t slow
down that you would never do in any regular routine move stress test.
Wings and tube did just fine…didn’t they?
 
WG   :-)
 
 
 
 
 
 
 
 
 
-----Original Message-----
From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]
On Behalf Of Gray E Fowler
Sent: Monday, October 04, 2004 8:31 AM
To: discussion at nsrca.org
Subject: Re: Applying poly glue-a different opinion
 

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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