Covering with Silkspan
By Jim Ryan
Introduction: Over the last 25 years, the choices of covering materials for R/C models have expanded steadily. So why is it that some "old timers" still cover models with old-fashioned silkspan? Well I'm not exactly an old timer, but I can think of four reasons. Covering with silkspan is: 1) easy, 2) cheap, 3) fun, and 4) beautiful! Silkspan is lightweight, accepts nearly all paints readily, and will never ever sag, bubble or wrinkle. It goes on just as easily over either sheeted structures or open framework, and even repairs are a snap.
While generally used on smaller models like Old Timers, 1/2A glow and small electrics, silkspan is also an excellent surface prep for sheeted surfaces even on giant scale warbirds. It's an excellent choice for applications like simulating fabric-covered control surfaces. Be forewarned though, silkspan is easier to tear or puncture than most modern coverings, but it's also extremely light.
Please note that the nitrate dope used for applying the silkspan is not fuel-proof. If the model is going to be exposed to fuel, you'll have to use a fuel-proof paint like butyrate dope, epoxy, polyurethane etc. over the nitrate. For electrics or gliders, you can use whatever you like for the color coats. Another nice technique is to color the silkspan with household dye before applying it. The options are almost endless.
1. Materials: The required materials include: sheets of silkspan (for all but the smallest models, I use medium or heavy material), nitrate (not butyrate) dope, dope thinner, water, 2 or 3 brushes, talcum powder, 240 and 400 grit wet/dry sandpaper.
2. Surface Prep: After finish sanding the airframe and removing all dust, brush 2 to 3 coats of 20% thinned nitrate dope on all surfaces that will contact the covering. Sand the airframe lightly with 240 sandpaper between coats. You want a reasonably water-proof seal so the balsa will resist warping when you apply the wet silkspan.
3. Application: I recommend starting with the bottom of the wing, as it's the easiest surface to cover. Lay the wing on a clean work surface and trim a sheet of silkspan to size so that there's 1 to 2" of excess around the perimeter. Brush the sheet of silkspan with water until it's completely saturated. This will cause it to swell and wrinkle.
4. Trimming: Start lifting and smoothing the silkspan until all wrinkles are removed and it's pulled fairly taut. The wet brush will help you to force bubbles to the edges. Be careful not to tear it, but even wet, it's surprisingly tough. Wet a piece of 240 wet-dry sandpaper, and sanding on the downstroke only, feather away the excess silkspan. You'll find you can easily work around compound curves and can wrap the material around the leading edge and wingtip.
5. Doping: Once the silkspan is trimmed, and while it's still damp brush on a coat of nitrate dope that's thinned 50%. The dope is this highly thinned because you want the thinner in the brushed-on dope to partially dissolve the dope that's already on the bare balsa. This will bond the covering to the airframe. At this stage, it will look mottled, whitish and ugly, but all will be well once it's dry.
6. Continue Covering: Continue to cover the rest of the wing and then the fuselage following steps 2 thru 6, overlapping the successive pieces so that there are no gaps. If you tear a sheet or can't get it to lay properly, just lift if off and try again with a fresh sheet. Apply a second coat of thinned dope to the covered airframe to make sure the silkspan is fully saturated.
7. Additional Coats: Once the entire airframe has been covered and doped into place, allow it to dry thoroughly. As it dries, the silkspan will turn a uniform white color and will pull nice and taut. Next, brush on 2 to 3 additional coats of dope, sanding very lightly between coats with 400 or finer sandpaper. Be careful not to sand through the silkspan, especially over open frame.
8. Mixing Filler: To fill the weave preparatory to painting, mix regular talcum powder into your thinned dope until you have a slurry. You'll have to acquire a feel for how much to use; too little and it doesn't fill very well and is harder to sand; too much, and the filler will be porous and won't take a smooth paint surface. The right mixture has the consistency of watery wallpaper paste.
9. Filling: Brush the talcum/dope mixture onto the model and allow it to dry. Work quickly, as the mixture will set up in a hurry.
10. Final Sanding: Lightly sand the airframe. Note that the talcum is not only easy on your skin, it smells lovely as well! If you can still see rough spots, repeat steps 9 and 10. That's it! The airframe is now primed and ready for the paint finish of your choice.
11. A NOTE ON REPAIRS: When patching holes or repairs, tear the silkspan patch instead of cutting it with scissors. A ragged edge is much easier to blend into the surrounding surface than a cut edge. Simply tear the patch to size, lay it in place, wet it, and dope it down. After the patch dries, apply 2 to 3 coats of thinned dope, sanding between coats. Then fill, sand and paint the patch. Wasn't that easy?
Conclusion: This covering
technique is lighter than nearly any film, will never wrinkle, and is quite easy
to do. There's no trick to it!