INTRODUCTION

About the time control-line started to become a very popular form of modeling,
a new type of covering material, called Silkspan, became available. It was
stronger than Japanese tissue, much easier to use, and lighter and easier to
finish than silk. Control-line modelers took to it strongly, and still use it
because of its many good points.

It, and other coverings that required dope for attachment and finishing, fell
into disfavor when high-nitro fuel proof plastic iron-on films came along. But
now the worm has turned. Electric power has now made it possible for us to take
advantage of the things we liked about Silkspan in the past: light weight, ease
of finishing, ease of repair, etc; not to mention the wonderful smell of
nitrate dope in the shop. Let me try to convince you to try it, particularly on
the new breed of small scale electrics (speed 400 types) where you need to save
weight but also want to be able to finish the plane in scale colors.

Advantages of Silkspan
1- Very light weight
2- Covering stays tight
3- Easier to apply than tissue or silk. It will conform to compound curves when
   applied wet.
4- Excellent as base or filler coat over balsa
5- Takes any kind of color paint
6- Costs less than film or synthetic
Disadvantages of Silkspan
1- Not as strong as plastic film.
2- Not high-nitro fuel proof
3- Not shiny-if you care
WHAT IS SILKSPAN?
Silkspan is a type of light, thin, flexible paper with a loose random weave and long fibres. Depending on the manufacturer it has quite variable weight, and may have little or no "grain". That is, unlike tissue, it does not tear straight in one direction and rough in the other.  You should try to find whatever grain there is though because, like tissue, it will shrink more with the grain than across it, so always cover with the grain spanwise.
The random long-grain weave gives silkspan something that tissue doesn't have: wet-strength. You can use it like cloth in covering. It will not fall apart when wet. Silkspan now comes in three weights, at least as Sig sells it: 00, which is the lightest, and is almost as light as tissue paper; GM, which used to stand for "gas model", is some heavier; and SGM, which is the heaviest. There used to be some called "rubber model silkspan" which was very light and was close to what Sig calls "00".Silkspan used to come in colors, and may still, but Sig now sells only white.
You should look around at some of the Free Flight or Old- Timer model supply houses for some of the colored stuff which may be imported still.
HOW DO YOU USE IT?
Silkspan can be used to cover anything that's wood, and maybe some other things too. While its primary use is to cover open framework, it can be used as a cover for solid balsa before painting. In fact, it is better for this than wood filler because it is lighter and basically smoother. The controlline stunt folks ( pardon me, Precision Aerobatic flyers) use silkspan as the first filler coat over balsa, and  they are the best airplane finishers there are, I don't care what anyone says.
You will need:
1 Some silkspan of the weight you need. I use mostly 00, and I have covered 1/2 A models up to a full size Buzzard Bombshell powered by
a Super Cyclone with it. It is a little delicate, so maybe GM would be better for large models. Now is a good time to talk about delicate.
You can put your finger through Silkspan, unlike Monocoat, but you have to give up something if you want light weight and a covering that
won't sag. 2 Nitrate Dope -- not butyrate.  The difference is that nitrate will stretch taut, and will stop stretching when it dries.  Butyrate never
stops shrinking so it can warp light structures weeks after you think you've finished. I buy my dope (and thinner) by the gallon at the
aircraft supply store at the local airport. If you don't have one of those, Sig sells it, but the price will be higher.
3 Nitrate thinner---please don't use "paint thinner" or some such. Get it at the same place you get the dope.4 Sharp scissors, sharp knife, sand paper of assorted grades, and dope brushes ( 1/2 inch, and 1 inch, ) 5 A heat gun, like you would use for Monocoat. This is a luxury; you don't
need it, but it's fun to use, and quick. 6 Dope "retarder"or plasticizer. Dave Brown sells it under the name of "Flex All". We used to use Castor oil. Some have used TCP. Whatever it is it is just an oil that won't dry out, at least for a very long time. It prevents the dope from becoming brittle with age, like people. That's it. You will need whatever finishing stuff (color) you want.
APPLYING THE COVERING
Open structure, like wings and tails: First mix some some dope, a pint or so, 50/50 with thinner. I almost never use any dope full strength, or any thinner than that. Paint the outline of the frame to be covered with the mix, and let it dry. Cut a piece of silkspan - grain going long ways- enough larger than the frame to be covered to grab ahold of. Some Silkspan has a smooth side and a "rough" side, so check for it and cover "smooth side out".Now the fun begins. Soak that piece of silkspan in water -in the bathroom sink is fine- and wring out most of the water. Now lay that wet silspan over the frame like a sheet on a bed. It will just lay there, unlike monocoat. Pull it out until it is straight, again like the bedsheet. You don't have to be too carefull;it won't tear unless you really pull hard.. Once you have the wrinkles out and the sheet is where you want it, paint dope
around the outline of the frame. Use your fingers to rub the dope gently through the silkspan onto the previously doped balsa. Rub the silkspan around the frame so that it is stuck down over and around the frame. You will be surprised how easy this is. And you had better get used to dope on your fingers. It won't hurt, is not toxic, comes off with thinner, will wears off soon anyway, and is something we old timers considerred part of the fun of modelling. Dope is not like CYA or epoxy. No matter what you get it on or in, it will always come off with nitrate thinner, even after it dries. Again remember to use only 50/50 thinned dope for everything.Now let the frame dry, and when it is, the silkspan will also be dry, and will
have shrunk taut, with the wrinkles gone.With a knife, or better yet fine sandpaper, trim the silkspan around the edges. Now, with dope on your fingers again, smoothe the silkspan down all around the edges.Make up some plasticized dope by adding Flex-All, or whatever, to some dope, in
the amount of one tablespoon per unthinned pint. Label this jug,  because you don't want to use this stuff for attaching covering. Slather this plasticized dope all over the silkspan and the frame, and let it dry. The covering should now start to look pretty nice, but don't touch it till it is thoroughly dry. Then sand the covering lightly with fine sandpaper to remove the fuzz. Then apply as many more coats of plasticized dope as you want. I use two to keep from adding more weight than necessary. You can use the heat gun to evaporate the dope solvents and speed up the process.That's it  You have a covered surface. Do the rest of them the same way. (I remnded myself of the Briish motorcycle repair manuals that used to say "reassemble in reverse order".)The shrinking silkspan can cause a light structure to warp. Hold the piece down with light weights until the dope is dry to prevent that. I use .45 Cal. lead bullets for weights, but that may not be politically correct now, so you may use whatever you want. Once the Nitrate is dry, you're home free.Light sanding between dope coats will improve the ultimate finish.If after all of your care in covering you still have a wrinkle, sometimes an application of full strength dope will stretch it out. If not, cut out the offending section and patch it.
Covering balsa:
This is done just like an open frame except you put dope-not plasticized-over the whole surface to be covered. Then lay on your wet silkspan and dope over the whole sheet, rub it in, and trim it.For compound curves like cowls, use a knife or razor blade and make slices through the silkspan in a fan pattern across the curve- not along it, before doping. Then rub the dope along the fan slices to get the silkspan to overlap itself as it goes around the curve. The natural ability of the silkspan to go  around curves makes this operation much easier to do than to explain.When the dope is dry, a touch with light sandpaper removes the overlap ridges. You can't do that with film coverings.Now paint on more coats of dope to suit your taste sanding between each. You don't need to use plasticized dope here because there is no need to keep the covering flexible.One thing to notice here is that you have totally filled or covered the balsa grain. You now have a perfect surface for...Painting or color coat:Nitrate dope and silkspan will take any finish known to man, I think. Because of its slight roughness anything will stick to it, and you will not need many coats to cover. You can use primer or filler if you want it to be shiny, but sand it all off to keep the weight down For color, one light coat of Krylon, for instance, will do for most of us. Obviously, if you have found some colored silkspan, your final finish will be the clear dope, and that stands up to weather just fine. I have eight models currently hanging in my  garage that are as much as 30 years old. Five ignition old-timers and three electric old-timers, and the covering still looks good on all of them.
A word about repairs:
If you do cut or puncture the silkspan covering a frame, it is easy to repair. Cut away the torn section, out to the nearest frame member or not, whatever seems right. The strength of the silkspan will prevent it from tearing any further.Cut a piece of new silkspan as much larger than the hole as you need to attach it with dope. You  can apply it either wet or dry. If its small, I generally apply it dry. Dope the patch on and let the dope dry. If you put the patch on dry, spray it with water to shrink it, and procede to refinish it as you did originally. With the heat gun you can complete a patch repair in 5 minutes, up to the color coats. This has gone on longer than I had intended. I originally wanted to talk about Tissue covering, too, but that has to be some other time. Unlike silkspan, however, there are many articles on tissue covering in the Free Flight journals. I will answer any questions if I can.  Also this article may be freely distributed.  Just credit EZONE, please.
Bruce Cronkhite  minton@worldnet.att.net
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