I can only tell you Steve what has worked for me over the years with that type of repair of a joiner or reattachment of two wing panels. Yes the original wing joiner that snapped may only be 1/8 inch but what I am suggesting above is to laminate two more joiners one in front of the spars and one to the rear of the spars that are also 1/8th inch and also Birch Ply (which will give you a total of eight plys).

And if I was short in material I would get it from my club mates for sure. Its a very common model builders material.

Yes fiberglas on a wing joint does help with strength but the number one joint on a wings integrity has to be that joiner at least in my book.

Take a look at this photo. It is taken on the flight test day of a wing that I modified for a cross country Cadet float plane that will carry a gallon of fuel. This day it is mounted on just a regular Cadet trainer with a .65 OS ax engine. This wing is from old airplane and the center section has be hack sawed in two and latter reattached in exactly the same way I described above with the same doubled Birch joiners in the front and rear of the original spars. There is fiber glas over lay but lite two ounce I think.

This wing also has been increased in wingspan by ten inchs at the tips. There are exactly eight pounds of sandbags taped to the center section of this test rig and the airplane weights seven pounds. It flew very successfully that day and what I would say is a pretty good test of the centersection joint. This wing has flown since on the final airplane now quite successfully and at the all up eight weight of seventeen pounds.

John