Words and photos by Bob Benjamin – firstname.lastname@example.org
A genuine Golden Oldie that’s perfect for today’s small electric airplanes.
NOTE: This article is required reading for the next installment of Master’s Workshop, Guillow’s Hellcat #4
Silkspan … what’s that? Simply stated, it’s a unique kind of paper that since the really old (pre-WWII) days of aeromodelling has been recognized as an excellent covering material for model airplanes. More specifically, it’s a specialized tissue made from woody plant fibers. It’s my understanding that most if not all of it comes to us from Japan. The material generations of aeromodelers around the world have called “silkspan”, a trade name of which I don’t know the origin, is very specific kind of tissue paper, but not all tissue from Japan is silkspan. You’ll recognize its subtly mottled appearance when you see it…and you almost certainly have already. “Silkspan” is the model airplane world’s name for teabag paper. “Japanese Tissue”, the lighter, finer-grained paper made from the gampi plant, sold on the US model airplane market under names such as Esaki, is a different product which demands slightly different application techniques.
Back when I got started in this business, during the late 1940’s and early ‘50’s, silkspan had long since been accepted as the default covering for all but the very smallest…and largest…model airplanes. In those days nearly all the kit manufacturers included a couple sheets of the stuff in every box as the recommended covering material, which everybodyknew was supposed to be attached, sealed and finished with airplane dope. The silkspan that was available to us then (and still is) came in three different weights/thicknesses. “OO” is very light…very similar to the heavier tissue products and good for little models with wingspans on the order of twelve to maybe eighteen inches. “OO” silkspan has another special purpose application which works well on larger models as well. I’ll talk more about that later in this article. “GM” (gas model) silkspan is noticeably thicker and stronger…and the most commonly used. “SGM” (super gas model) silkspan was also there on the hobby shop shelf if you were building a six-footer and couldn’t afford very-much-stronger (and much more expensive) silk. If you opened a kit from Sterling, Goldberg, Berkeley, Top Flite, PDQ, Midwest, Kenhi, or Veco, among others in those days, you expected to find a couple of two-by-three foot sheets of silkspan included. This arrangement continued to be common into the 1960’s and beyond, long after most model builders had come to regard those various plastic film products as the new default choice of covering. You will still find silkspan in all the larger Guillow’s built-up balsa kits today, and that happy anachronism is what is going to lead us into this lesson on one of the good old skills of aeromodelling.
Regardless of the grade of silkspan you chose, if you were an experienced model builder you had long since learned to apply your covering NOT just over the “open” parts of the model structure. Instead you covered every square inch of the model’s surface, including the “closed” parts like sheet balsa or carved balsa blocks. If you left any balsa structure uncovered/unsealed by silkspan you would pay the penalty of having a rough, porous surface that was nearly impossible to finish to match the “covered” areas. A colored dope finish might help disguise the contrast between the silkspan and the bare, open grain, porous balsa around it, but even adding and sanding multiple coats of various “fillercoat” products could not give you a finish base to compare with “silkspan over everything” and a couple coats of clear dope. Some guys learned the hard way that if you started that color finish by putting on enough coats of sanding sealer/fillercoat over the bare wood to truly hide it, the extra weight pretty much guaranteed a non-flying airplane. Indeed, using colored dope containing heavy pigments was something we learned to avoid when building smaller airplanes that we wanted to fly well.
No matter how you planned to finish it, like most of the other “pre-plastic” coverings, silkspan could be applied dry by sealing down all the edges of the piece you were working on and then spraying it with water, which when it dried would shrink enough to pull out most wrinkles. The alternate method, which most of us preferred, was to cover wet by pre-moistening the silkspan with water and then taking advantage of the extra flexibility and stretchiness that resulted to pull the covering tight and flat even over compound curves like the top surface of a wing. As the water evaporated the covering would shrink itself really smooth. Once you got used to working with the more delicate, fragile wet silkspan you were almost guaranteed a tighter, neater covering job. Regardless of how you stuck it down, a silkspan covering job was always sealed and strengthened by adding several coats of clear dope. Usually you stopped adding coats when the silkspan began to look noticeably shiny after the dope dried. Using more dope added more weight but even worse, it always threatened to over-tighten the covering and warp (twist) your structure. There are different types of dope as well as various different techniques for using it, but no matter…they all come with a REALLY STRONG SMELL. Even if you didn’t mind it everybody around you DID…and that is one of the best reasons why lots of model airplane builders stopped using dope-and-silkspan as soon as they discovered those new plastic coverings. Very recently a new product for sealing/surfacing silkspan (as well as other types of tissue) has become available to model builders. It does all the things we want the dope to do, but it DOESN’T SMELL and it doesn’t over-shrink. I will tell you more about this stuff later, but right now I want to talk about one more characteristic of silkspan that really expands its usefulness.
Silkspan, which is naturally white, used to be available in multiple pre-dyed colors as well. You could find red, yellow, orange, blue, green , black…or even checkerboard-patterned silkspan in red or black stocked right on the hobby shop shelf next the white kind. Can you imagine a model covered with, say, deep golden-yellow translucent silkspan sealed and turned glossy with five or six coats of clear dope, glowing in the afternoon sun at some grassy flying field? This is the effect that those transparent colored film products have been trying for years to reproduce. If you have ever seen a well-executed clear doped colored silkspan (or silk) covering, you will agree with me that they have not yet managed to get it right.
OK, enough of the history lesson. I have chosen clear doped colored silkspan as the covering/finish of choice for my ongoing series of electric RC conversions of Guillow’s traditional stick-and-tissue balsa model. I am going to show you how to use it a bit differently than we did in the past and in the process explain why learning to do it the new way is such a good idea.
There you have it boys and girls. An old classic that uses a few new tricks and products to get the job done better than ever. Be sure to check back in the next day or two for the latest Master’s Workshop installment, where all of this silkspan goodness will serve you well.