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I did another project using Zepron:
Summary:
The fastest-cheapest glue around (that's safe for foam) is 3M 77 Spray contact cement. The first step is to mask off areas you don't want glue to be on. The reason to use the minimum amount of glue is that glue is the heavy.
Spray twice as much as you think you will need. The reason? You are going to remove the tape and transfer the glue to the other piece of foam before it gets sticky. That means work fast because this glue gets tacky in less than a minute. The transfer should take less than a second and immediately separated. While apart they will get tacky. During that time spray the balsa; both sides. The tape on the balsa, in the picture, was a misguided effort to keep the edges of the balsa from getting glue on them. Don't bother. Just lightly spray both sides and wait for maximum tackiness before placing the balsa on one of the foam sheets. Notice that the balsa goes on the 25% point of the cord. 
If you look carefully you can see the shine of the adhesive on the trailing edge. There's glue on the leading edge too of course. If you've never worked with spray
contact cement before, the best advice is to plan your steps so the glue doesn't have to wait more than a few minutes before assembly.
This picture shows a piece of tape holding together leading edge. Contact cement is famous for slowly untying itself if the tension is constant. The tape insures that it
won't do that. It is not necessary on the trailing edge. Zepron sands beautifully so just before you tape the leading edge sand it. The contact cement holds it together
long enough to sand it. And after a day, the contact cement is hard enough to hold it together without the tape, but the tape adds a bit of toughness to the leading
edge, so leave it on.
Here in the last but most important step is the application of 1/64th ply to both sides. This can be done with spray adhesive using masking tape and paper towels as
a mask, or you can use epoxy. Either way it's messy. But don't leave off this step. The ply adds more stiffness than the balsa inside. In fact, you should think of the
balsa as just a spreader to give it an airfoil. The strength is in the ply.
More Info:
I'm a strong believer in flying tails. I never use elevators. And I pivot the tail from above the stabilizer. That's what those bearings are for in the picture below. If you
are going to put elevators on the tail, I urge you to put a strip of balsa between the sheets of foam on the trailing edge. And make it exactly the same size as the
stabilizer; that is, the hinge should be at the 50% point of the cord. The elevator itself should be built exactly like the stabilizer. A full length of white masking tape
makes a wonderful hinge. If you are unaccustomed to having such a big elevator, the one thing to keep in mind is the need for a tall control horn. And for heaven's
sake don't buy a control horn. It only takes five minutes to make one. The one below is made of 1/8th balsa and 1/64th ply. You make the ply wood first, then cut it
out.
This method of building is recommended for anything larger than 2" cord. For a stabilizer on a home built indoor airplane where the stab is less than 2" in cord, use a
single sheet of foam. But don't forget the 1/64th ply on both sides. It is thin enough to not be an aerodynamic drag, but thick enough to be an excellent spar. But it's
only a spar if there's a matching piece of 64th on the other side. The picture below is of a swept back tail of about 12" span. (Those numbers are in millimeters.) The
weight on the single sheet tail is 1/8th ounce. The weight on a two skin version is obviously twice as much.
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