Q-102: I am somewhat of a novice, but have built six airplanes so far.
Occasionally I have a small problem with the covering lifting a little on the
edges. I thought it would be a good idea to put some sort of clear finish on
these edges to prevent the covering from coming loose. I have seen references to
using a urethane-based product to do this. Is urethane fuel proof? I tried a
clear finish put out by one of the covering companies and found that after two
months the finish had still not dried. What kind of advice can you give me?
A-102: I have to admit that I personally have not had to resort to a
paint-type product to help seal the joints or seams of my iron-on covering
material. I’ve asked several experts who tell me they have successfully used the
old two-part epoxy clear paints such as K&B Super Poxy and Hobbypoxy
successfully. I’m not sure if either product is still on the market.
Another possibility is clear nail polish, which comes with a brush in the cap
making it easy to apply. It's hard to say just how fuel proof this will be. I
hate to throw a little dig in here, but since I only fly electrics, I never have
a fuel-proofing problem. One expert claimed he had successfully used thin
cyanoacrylate cement, but sparingly.
My honest feeling is that iron-on covering seams or joints come loose because
the correct amount of heat was not employed at the time of application. I’ve
seen modelers tack on the covering and then quickly shrink it out tight on the
model structure. After doing this you should increase the iron heat somewhat and
go back over all of the joints. If I get a stubborn joint I’ve found it helpful
to clean off the surface with some denatured alcohol. Let it dry and then try
applying heat once again from my iron. Usually when the joints and seams are
sealed one more time, they never come loose.
I’ve explained many times before that I actually use two irons when covering my aircraft. One is set for a lower temperature to just tack the covering in place. A second iron is set for a higher temperature and is used to really seal the material to the structure and to itself (the joints!). After you get everything anchored down you can shrink the covering until the wrinkles are removed and the surfaces are nice and taut. But the key to all of this is getting enough heat on the joints (or seams) in the first place.