Apply a stits lite finish

Model Airplane News, Jan 2002 by Benjamin, Bob

MODEL AIRPLANE NEWS

HOW TO

An easy-to-apply, scale covering material

Today's model airplane builders have a vast assortment of hightech products to choose from when planning a new project. From the most sophisticated computer radio to the smallest bottle of glue, RC modeling truly is a space-age activity that involves many products derived from full-scale aviation. This is especially true of covering and painting materials. Using these products requires patience, but the finished projects are well worth the effort.

Having tried nearly every covering product on the market, I now prefer to use the Scale Stits system from F&M Enterprises for all but my very smallest and lightest models. Stits covering was developed exclusively for model airplane use by F&M's Chip Mull, and it's available in weights and quantities that are appropriate for models. With some minor modifications, the techniques used to apply Stits covering will work with any of today's other popular model fabric and paint products. The model featured in this article is my Dynaflite 1/4-scale Fly Baby.

* Preparation. Regardless of which product you use, your real first step toward a good covering job is to prepare the model's structure by sanding smooth every part that will come into contact with fabric. For this, I use a sanding block and increasingly fine sandpaper grits until everything is satin smooth. I follow this with a wood sealing product that's compatible with my chosen finish. I use two coats of clear, non-shrinking nitrate dope; when dry, it's very easy to sand, and it accepts Stits products nicely. Stits Poly Brush primer will work well also. The idea is both to seal the bare wood against moisture and to improve the fabric's adhesion. When all of that has been done, cut your fabric pieces just a bit larger than each structure to be covered.

With the fabric in position on the structure to be covered, fold back a section and apply Poly Tak adhesive along 6 to 8 inches of the structure's outer edge. Using your thumb and fingers, press the Stits fabric firmly into place, letting the adhesive penetrate the weave. After a moment, the adhesive will hold the fabric tight. Work around the perimeter of the structure, and wrap the extra fabric over its edge to the other side. Pull it and press it to smooth out all but the smallest wrinkles; they will be eliminated with heat later.

Having covered one side of the surface, repeat the process for the other side. You must seal the second overlap down over the first layer with more Poly Tak adhesive. Use finger pressure to smooth down the overlapping layer. This bond will never slip, and the double layer of fabric will reinforce the structure's edges.

* Heat shrinking. You may damage the fabric if you overheat it, so don't use a heat gun; instead, use a covering iron set to the temperature suggested in F&M's Stits instructions. I use a calibrated 21st Century covering iron from Great Planes set at 225 to 250 degrees Fahrenheit. I lightly touch the fabric with the iron to smooth out small wrinkles and to seal down any loose areas around the structure's perimeter. Poly Tak adhesive is heat-activated, so heating helps to bond the fabric to the structure.

The instructions recommend that you shrink the fabric in several stages, increasing the temperature by 25 degrees at a time. This ensures uniform shrinkage, and it doesn't tighten the fabric so much that you damage the structure. For stubborn wrinkles, you can increase the heat to up to 350 degrees for more shrinkage, but do this only where absolutely necessary. The Stits fabric is extremely stable and will stay taut.

When all the fabric covering is in place and has been shrunk tight, seal it with a coat of Poly Brush fabric primer. It's the equivalent of a clear dope coat, and it seals the fabric so that it's ready for subsequent color coats. Poly Brush does not, however, help to shrink the cloth.

* Fuselage. I modified my Fly Baby to include the classic fabric fairing between the vertical tail and the fuselage turtle deck. Using Stirs fabric, what you might have thought impossible is actually easy. Begin by covering the horizontal stabilizers (if they're permanently attached to the fuselage), and then cut a piece of fabric that's large enough to cover the entire fuselage side from the tail to the firewall. Extend it above the top stringer and below the bottom longeron. The fuselage bottom will be covered with a separate piece; you may do this before or after you've covered the sides and top. Cut a slit in the cloth for the horizontal tail to slip through, and start to attach the fabric to the fuselage there. Next, glue the fabric down all along the vertical fin's trailing edge (TE), and then along the lower longeron, all the way to the nose, pulling the fabric as taut as you can. Now attach the fabric to the curved part of the dorsal fairing, and work along the fin's leading edge (LE) and forward along the top stringer. Wrap the top edge around the top stringer and use extra Poly Tak to secure the fabric; this forms the base for the fabric you'll attach to the other side. The secret here is that you must pull and stretch the fabric as you go to make it conform to the curve you want. This takes patience and some practice, but the result is worth it.