2007 Scale & Non-Scale Events


Elmer's Pro Bond Polyurethane Glue

Every so often there comes into our experiences something new and exciting that gets the juices flowing again. In years past it has been iron-on films, CA adhesives, "reliable" radios, etc. The latest item to hit my "excite" button is Elmer's Pro Bond Polyurethane Glue.

I have just finished skinning the wing on my GBZ using this material. It was used initially to install the spars and seemed to work so well that I did some experimenting on scraps to see how it would perform on the sheeting task. I used varying amounts of glue and varying amounts of water to determine the amounts of each that worked best. Even though the glue will foam up by itself, it seems that water aids this process. I found that stringing beads of glue approximately 3/4" apart and spreading it to a very thin film on the sheeting with a plastic scraper worked quite well. Because of the foaming action, it does not take much glue to generate a very good bond. The water was misted on the foam wing with an atomizer. The amount of water was just enough to wet the surface well but short of beading up. (Don't use water in the summer!)

There is a weight savings in using this type glue over using epoxy because not nearly as much is required to do the job. Because of the expansion of the glue it is not necessary to use as much glue to fill any voids and assure a good joint. Also, because of the expansion, the wing core does not need to be sanded as smooth, but just freed of any high spots with a long block sander.



As is usually the case with spar slots in foam wings, the slot was rather oversized compared to the spar. This usually causes us to fill the void with epoxy, adding unnecessary weight, just to get a good attachment. The foaming expansion action of the Polyurethane glue is tailor made to solve this problem.

The process is done to one side of the wing at a time. The wing was placed in one of the core shucks and a bead of Polyurethane glue was placed in the bottom of the each slot. The spar was heavily misted with water and placed into the slot. A strip of 1" masking tape was then placed full length of the spar to confine the expansion to the slot. A strip of waxed paper is then placed on the wing and the top shuck placed atop that. This assembly is then weighted down much as is done with an epoxy sheeting process. I use steel weights that evenly distribute the load over the assembly. The combined weight is approximately 150 lb. Because I am in no hurry, I leave this to cure overnight.

Upon removing all the weights, shucks, etc. I found that the glue had filled the voids and even sent "runners" into the pores of the foam itself for about 1/2 inch. This gave additional strength to the bond of the spar to the foam.


Applying the sheeting on this wing was made easier by the fact that it has open bays with cap strips. This allows the sheeting to be applied in smaller pieces than a fully sheeted wing. The individual sheeted areas were applied one at a time. The sheeting was positioned on the wing core and held in place by masking tape. The tape was located such that it prevented the expansion of the glue into areas it was not desired. The sheet edge at the spar was taped full length to prevent the glue from entering the bay area.

After locating the sheeting it was folded back over the tape and Polyurethane glue applied as noted earlier and spread to a very thin film with a plastic spreader. Water was then misted on the foam core where the sheeting would lie. As soon as this was done the sheet was folded back into place, waxed paper separator installed, top shuck installed and the assembly weighted as noted earlier. Again, because I was not in a hurry, this was allowed to cure overnight. There is probably some reduced amount of time that could be allowed for the cure, but I will let that be someone else's determination.

After the large sheets were in place the cap strips were installed with Gap-Filling CA that does not attack foam. The bays were then lightened by "pocketing" out the individual bays on each side of the wing to leave a 3/8 inch thick web in the middle of the wing. The web left in the wing affords some amount of transverse strength remaining in the wing while at the same time lightening the wing and preventing the covering from contacting the foam core. The "pocketing" was done with a soldering gun to which #12 or #14 copper wire from household "Romex" wiring was installed. The wire was bent to the desired shape to just fit between the cap strips to the desired depth and ledges bent in the wire to ride on the cap strip. The cap strips were protected from the heat of the wire with two layers of masking tape.

The remaining leading edge and end caps were then applied and the wing sanded for covering. If I were to have done a fully sheeted wing, I would follow much the same process except for positioning the sheeting on the core. I would likely have used masking tape in selected locations to assure the sheeting remained properly located. There is a limited amount of time allowed to spread the film and water before the glue starts to react, but I had no problems at all with the above. I believe any type sheeting could be applied in the time allowed. It is doubtful that both sides could be done in one setup. Besides, if you are in a hurry you are in the wrong hobby anyway.

CAUTION: When hot-wiring the bays of the wing it is advisable to provide generous ventilation as melting polyurethane does give off some toxic fumes.

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