How To Use “Good Old Silkspan” as a Covering and Finish Base (Part 1)

In one of my earlier blog series I offered a detailed discussion of SILKSPAN…what it is, why I use it, and so on. If you missed this, check it out under the FlyLine Great Lakes Trainer and Return of the TigerKitten categories on the homepage. The story this time is about the specifics of USING the stuff on the Dornier 217 project you have already watched me build. One thing you should always keep in mind about silkspan is that it is not just for covering “open structure”…the spaces between the wing ribs, for example. Silkspan is one of the best materials you’ll ever find to provide a light, reliable finish base over sheet balsa, carved/curved solid balsa, etc…like the center section of this Dornier’s wing. Even though there’s no open space to cover, there’s plenty of balsa grain to fill (unless you want to look at it forever). Trust me, a layer of silkspan with several coats of dope is a quicker, stronger and lighter way of making the wood grain disappear than brushing on and sanding out all the EXTRA coats of dope and filler/sealer it takes to do the job without the silkspan base. Here I have chosen the area of the underside of the right wing between the centerline and the right engine nacelle to begin with, and cut a piece of silkspan large enough to cover the entire working surface with an inch or more over overhang all around.

Applying each piece of silkspan wet (with water) is the only technique that makes sense to ME when working over solid (sheet covered) surfaces. Unless dry silkspan is absolutely wrinkle-free not even dope will permit it to lie flat, and even then it will have no ability whatever to be worked around any sort of compound curvature. A light misting with water makes these issues go away. I like to work over a clean towel…the cloth absorbs any extra moisture and provides a surface that won’t catch or tear the silkspan.

This is where you get to learn to use a delicate touch. I have laid out the wet silkspan lightly across the area I’m going to attach it to. There should be some overhang beyond the “free” edges (like the leading edge) but don’t let the material run up over a concave area like the transition between the wing surface and the engine nacelle. That would create an opportunity for the silkspan to lift and “bridge the gap” as it dries and shrinks. Once that happens you must cut away the detached silkspan and start over or accept a “bubble” in the covering and finish that will never go away.

Here I am brushing a generous, wet coat of clear nitrate dope through the silkspan and into the underlying balsa surface, starting in the center of the working piece and always brushing OUT toward the edges. This is where you get to work out any last little wrinkles or loose spots with the brush.

Work enough dope into the silkspan and out past the edges of that piece to wet right through it and smooth out EVERY imperfection…the covering must be snug and tight against the balsa surface now or it will never be right.

Where the extra covering overhangs an edge, slit it into a fringe and dope each short section smoothly over and around onto the opposite surface. This will end up creating an overlap that measurably increases structural strength and protects the balsa structural edge against dings. Not leaving any narrow bits of bare balsa surface uncovered will also make the final job of finishing a lot easier.

Now we will cover an “open” structure ( the left wing undersurface) where the silkspan performs its primary function of spanning and closing up areas like the open spaces between these ribs. I cut a working sheet of silkspan to size and water sprayed it just as with the last piece. On this open structure I’ll apply it differently. I start by spreading the moist sheet as smoothly as possible over the working area. There are going to be some waves and slack areas, but there must not be any sharp creases or overlapping wrinkles.

Next I’ll pull and stretch one edge of the sheet…I started with the inner end of the wing panel… as smooth and tight as I can get it and dope it down to the balsa structure all the way from the LE to the TE. I will then let that doped area dry for a few minutes, until it’s secure enough that I can pull gently against it from the opposite end of the sheet (the wingtip in this case) without it’s moving.

When I do that it looks like this.

With the sheet stretched smooth (not tight) spanwise I’ll fold back the outer end, apply a wet coat of dope to the structural outline, and then fold the covering back in place and stretch-and-pull it snug around the wingtip circumference.

The next step is to lift the still-loose leading and trailing edge portions of the silkspan in turn, six inches or so at a time, brush a wet line of dope along each section beneath the covering, and then pull-press-smooth the silkspan down tight against the balsa. (Remember that the silkspan must still be still moist at this point. If it should start to dry enough to lose that “limpness”, mist it with more water.)

Here I’m adding some extra clear dope to the opposite surface to wet and adhere the edges of the overhang. Remember that we’re working to create an overlap of the covering here.

This is the left wing lower surface with the still-damp silkspan fully doped into place. I have cut various slits and fringes as necessary (like at the corners of the aileron well) to permit every loose edge to lie down flat against balsa structure.

I finished covering the rest of the wing the same way. Always cover the upper and lower surfaces of a wing (or tail surface) together so they dry out simultaneously or you risk putting uneven stresses on the structure. My next step was to start covering the engine nacelles. These are entirely balsa sheet structures, so we’ll go back to the technique I first used and work out from the center of each sheet of covering. Here we are dealing with significant compound curvature, so I have kept the size of each working piece small enough that I can stretch-and-smooth all the way out to each edge before I trap any wrinkles. This is the bottom-front of the right nacelle. I’ve learned from experience that spanning about that much curved surface is close to the limit of the wet silkspan’s ability to stretch and lie flat.

I’m working the silkspan as far as possible around the nacelle front before going near it with the dope brush.

More of the same. This assembly is upside down. Some of that “top” edge will end up fighting me by wrinkling and I’ll trim it off along the midline of the nacelle so a separate sheet can provide the “stretchability” necessary to deal with the opposite (top) surface.

Now I can dope the working sheet down tight and smooth around the nacelle surface. You should be able to see where I made that cut I mentioned in the last frame.

Now I’m working on the bottom/center of the nacelle. I’ll split this covering job along the bottom centerline to avoid fighting both sides of that complex curvature at the same time.

Once I’m satisfied that the piece of silkspan will permit me to work it out-and-down until it lies flat, I can start doping it in place.

Partway along the nacelle I encountered a place where the wrinkle wasn’t going to give up, so I grabbed a sharp blade and cut a slit to create two flaps that can overlap and relieve the stresses that are causing it. Here I’m using forceps to get hold of one end of the section of wet silkspan and get it positioned smoothly in place.

This is another look at the same process. This sort of thing isn’t difficult once you accept the need to do it…and learn not to be in a hurry!

Here you are looking at the same nacelle from a different angle. This is what “putting on a wet coat of dope” should look like.

I covered the upper rear wing-to-fuselage fairing with a separate piece of silkspan. Here I am positioning the wet covering on the structure.

I don’t want the silkspan to reach across, or “bridge” the tight angle where the fairing meets the wing. This would just about guarantee bubbling and lifting of the finished covering job. I’m using a nice sharp No. 11 blade to trim around the perimeter of the fairing so I can remove the portion of the covering that would cause trouble.

Now I can do the “full, wet coat of dope” deal here. At this point what needs to happen is that the silkspan…still wet with water AND dope…is attached with no bubbles, wrinkles or creases and that I have it saturated with as much dope as it will hold without sags or runs happening. The intent is to fill the weave of the covering and let the wet dope penetrate the balsa surface as well.

I’m doing the same thing here with the front/upper portion of the left nacelle. This piece of silkspan is still dry…I’m test-fitting it.

The wet silkspan is in place on the nacelle. I have already trimmed it at the right where it meets the wing leading edge to avoid bridging and I’m working it smooth around the nacelle before brushing on any dope.

Time for dope. Just as before, it needs to be a thorough, wet application.

A small, separate piece of silkspan is the best way to cover the top surface of the wing where the rear edge of the nacelle blends in. I have pre-cut the radius in the covering to fit the curve of the structure.

That piece of silkspan also goes on wet. No dope yet.
That’s all for this session! I have to finish up another blog on the Stinson SR-9 project and then I’ll be back for some more silkspan work.