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The Hows and Whys of Silkspan Covering

by Bruce Cronkhite.

The Hows and Whys of Silkspan Covering
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 furt
her. 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.

Send comments to:
Bruce Cronkhite
minton(at)worldnet.att.net

Discussion

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Old Jun 27, 2007, 10:14 AM
rao
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Joined Jun 2007
1 Posts
Thank you for a wonderfully informative article! I just covered my own design scratch-built 36" electric with Silkspan, and your article provided the inspiration and directions. I especially liked your encouragement to use fingers to work with dope.
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Old Jan 20, 2012, 10:44 AM
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Silkspan and fuel proof dope

Correct me if I'm wrong, or if no one makes the fuel-proof dope anymore, but by covering a model with silkspan and "painting" with fuel-proof dope, you can use silkspan on a high-nitro model.
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Old Jan 20, 2012, 11:47 AM
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I keep seeing the recommendation to wet the silk(span) first, stretch it out and dope the wet silk(span) onto the structure.

What I remember reading a long time ago is to place the silk(span) dry over the structure, dope the edges, stretch the silk out and let the dope dry. Then spray the silk(span) with a water bottle and let it shrink. At the time, it seemed to make a lot of sense since I did not think you could glue down wet silk with dope.

Is there any disadvantage to the dry-silk application method I described? It work very well with silk and AeroGloss dope back in 1962.

Alan
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Old Feb 12, 2012, 05:31 PM
A posse ad esse
United States, FL, Tampa
Joined Feb 2011
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Al

I think the advantage of laying down the silkspan while it's wet is to allow an easier way to work around curves that could cause wrinkles if done dry. Either method works, just depends on the preference of the builder and whatever tricks they have learned over the course of time. Don't recall why or who taught me, but learned to lay it down wet many years ago when I first started building with tissue and dope. Since then, have done it the way you described, too, with good results.
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Old Feb 12, 2012, 07:15 PM
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Silkspan as well as silk tend to expand when wet, so once it dries after you apply it wet, it should be tight. Also much easier to pull wrinkles out.

John
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Old Feb 13, 2012, 08:25 AM
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I've used silkspan, all the way up to the 80's was the last time. I remember applying it "wet" to the framework... then lifting it and brushing dope just underneath it.. then waiting a few hours and it coming out drum tight at that point.

What I don't remember is "rubbing the dope in"... haha .. but ok.. if you like getting your hands/fingers in the cake mix.

It took about 4 coats of dope to get the silkspan to look decent.. once done though and painted(using colored dope)...it actually came out very nice. I used it back in the day when I didn't have funds to trot to the hobby shop for film covering.. and dope was cheap.. not to mention.. most kits came with silkspan. Good stuff.. it takes me back to a happy place.
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Old Feb 13, 2012, 11:38 AM
B for Bruce
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First off- Al, if you're still reading in here I sense some confusion over silk vs silkspan from your use of brackets. They are most definetly not the same things at all. Silkspan is a paper product which is much like that used for making teabags. Silk, of course, is a fine woven material.

For me I always applied my silkspan damp. The frame would be doped and "de-fuzzed" with two or three coats of dope. Then the dampened silkspan laid over the frame. To bond it down I used pure thinner to soak through the wet paper and soften the underlying dope to achieve the bond. And yes, some rubbing down to aid with sticking to the dope below the paper helped a lot.

As mentioned doing it wet made it a lot easier to pull out wrinkles or slightly stretch the silkspan around come MINOR amount of compound curving. Doing it dry the paper seened to have a mind of it's own and just would not lay flat over the framework well at all.

Using thinner to re-soften the dope on the airframe greatly aided in avoiding the massive blushing that would come from applying actual dope to the wet tissue and then smooshing it around with your fingers. So this made the use of clear dope and color tissue trim over the airframe a lot easier if that's the look you wanted.
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Old Feb 13, 2012, 05:26 PM
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I'll have to try that on the next plane I cover. While the blushing has never really concerned me as I usually air proof with a coat of dope or two after everythign is stuck and dried, but it'd be nice to not have to.
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Old Feb 13, 2012, 07:49 PM
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Quote:
Originally Posted by BMatthews View Post
First off- Al, if you're still reading in here I sense some confusion over silk vs silkspan from your use of brackets. They are most definetly not the same things at all. Silkspan is a paper product which is much like that used for making teabags. Silk, of course, is a fine woven material.
Definately agree. If I'm not mistaken though, silkspan, while being a paper based material(pulp wise).. does have a "silk" content of a fiberous sort... much like todays "high-end" paper/stationery can have a few different additives.. such as cotton, or yes, even silk.

Now.. to what degree, or percentage is the content of silk....I don't know.. and it might even vary depending on manufacturer.... but something in my memory banks is telling me this is so.. if I'm crazy please tell me too..

Does anyone remember "Sil-ray".. haha.. I wonder if I'm stirring any memories out there? ......
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Old Feb 13, 2012, 08:08 PM
B for Bruce
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Sil-ray? Anything like Silk-Spun Coverite?

I'm not sure what sort of fibers are in the stuff but I also read the same thing that silkspan is a fiber bearing paper. Not a clue other than the "silk" part that perhaps some of the early versions may have used some silk based short fiber "floor sweepings" or "machine floss" or some such thing....
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Old Feb 13, 2012, 08:20 PM
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My memory is vague, as I was probably about 13 or so.. and I'm thinking that was the product's name.. and it was manufactured/distributed through another name.. kind of like Monokote is to TopFlite.. It was a silk-rayon covering, applied identically to silkspan.. I used it on one plane..which was the Midwest Magician, CL plane. I might have an old label here.. I'll gander around.. I know I've seen one around here in the last few years.. but.. yeah.. now I would think that would be considered a fabric based covering....

Actually I've wanted to do either a fabric, or silkspan plane for a while now.. we're talking a whole set of extra steps though.. unless I went with an iron on fabric...

Too bad you can't iron on silk-span.. haha.. hey.. I wonder if you could? Using either balsa-rite, or something like that?? hmm... wet the paper... the heat from the iron would activate the balsa-rite... hmm.. now I am crazy..

Enough of me, I don't want to take the thread to far off topic...
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Old Feb 13, 2012, 11:17 PM
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Silk-rayon. I would like to get my hands on some of that. How much abuse can wet silkspan take? Can it be repeatedly crinkled and rinsed?
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Old Feb 14, 2012, 07:27 AM
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After many days of no thread activity, I just saw all the replies - thank you.

My early experience was with silkspan and dope on a couple of free-flight models but my later covering choice (before film was available ) was silk and dope on single-channel and galloping ghost models. After these many years, my memory must have blurred the distinction between the two.

My question was directed more toward the application of silk than silkspan since I would like to build a seaplane (flying boat variety) and cover it with silk and dope instead of film (mainly for long-term water proofing and partly to re-awaken the skill).

Alan
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Old Feb 14, 2012, 08:11 AM
Egads! It's a GIRL!
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Well, Al, I can't say I have much experience, yet, but I am having fun with my own "silkscapades". It's a really stinky process, but it's not causing me much trouble. But I am also using entirely nitrate dope, since the plane I'm working one will never see fuel of any sort.
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