How are the skins applied?

The foam airframe parts are systematically covered with individual skins that are adhesively attached to the foam.    We create the skins in large sections whenever possible, for example, the T-34C's half wing top is one skin and one side of the fuse consists of two skins with their joint occuring at a scale location. The molded skins have reference marks to guide final trimming. The skins are .010”-.020” so trimming is easily done with scissors or xacto. 

A critical part to skinning the model is using the right glue. We performed numerous tests to evaluate the best adhesive to bond the skins to the foam/polycarbonate. The preferred glue would need to: 1) withstand high surface temperatures (over 180 degrees F), 2) provide a sufficient bond to the foam and polycarbonate, 3) be flexible and 4) be available yet still affordable. Over 20 types of glue were tested and the best performing adhesive turned out to be outdoor caulking/sealant - the type used to seal doors and windows. Available at hardware stores for less than $5 for a big, 10 oz. tube this glue can be found just about anywhere and compared to hobby glues it is a bargin. A caulking gun is needed to apply the adhesive, these cost about three bucks. This adhesive spreads easily with a putty knife, there is a long working time (to position the skins) and after a 24 hour cure time bonds the skins sufficiently to the foam. Clearly, the bond strength needs to withstand the force of the airflow over the model during flight and this adhesive is excellent for this. Ideally, the adhesive should also allow one to delaminate the skin should a repair be needed. The window, door and siding sealant we use (PL brand, Polyurethane), allows delamination of the skin with about 10 pounds of pull force and most of the sealant sticks to the skin - this leaves a relatively clean surface to re-apply the skin or apply a replacement skin.

The process of applying the skins is striaght forward. To skin the wing for example, you would first apply the adhesive to the upper surface (it gets applied in long beads, like caulking tile, then spread with a putty knife to an even thin layer).  The top skin is positioned in place (you have lots of working time with the glue) and the skin is rubbed down for good contact with the glue and stab. In the photo below you can see the skin (with the green protective film) adhered to the foam wing. Rubbing down on the skin squeezes out the trapped air and this is evident when the adhesive (white in color) can't be seen as shown in the bottom of the photo. The rub down process on the wing takes about 2-4 minutes.

The bottom skin is applied the same way and then the wing with skins attached is placed in its foam saddles to cure overnight.  Then an overlapping leading edge skin would be glued in place followed by attaching the wing tip skin that overlaps the other skin's outer edges.

 

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