Willy Nillies J3 Cub Build


Elite member
I'm still working on sanding and finishing in preparation for covering. But I remembered a very good tip that I learned a long time ago that has saved me a lot of time and effort at this stage which I wanted to share with you guys.

LOL. That definitely is a “quick” method of covering so you can get to your next project.
Apparently, I'm not allowed to start another project until I clean all the other junk up.

So I've finished sanding down the plane. The balsa is soft enough that I just use 400 grit and not bother doing it in stages. I sand down all the sharp edges and corners so that they will not cut into the covering. That includes the cutouts as well, gives everything a soft rounded feel. In particular, I make sure to radius all the ribs.

I have four fillers that I use. Hobbylite to rub into scratches and pinholes. Elmer's for gaps and areas that need to be built up like the slot and tabs. Super Fil for fillets. This was rather expensive to get from Brodak so I'll probably get it from Aircraft Spruce next time. The red acryl filler is for the covering.

The wing sheeting fit rather well so I did not need much filler. I did take off a lot when sanding it flush, so something to take care when handling.

I also attached the elevators together using thick CA and filler. I installed a magnet for the battery hatch and used scrap music wire on the hatch. Finally I installed the pushrod, though I'm not sure if they should cross or not just yet.

Once the fillet and elevator fillers cure, I'll give them a quick sand and start covering tomorrow night.


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Now for covering, which I'm sure everybody already knows how to do but I'll go through it in case someone new comes across the thread.

For brushing, I am using 60-40 mix of 60% thinner. I start by brushing nitrate dope on the balsa everywhere that the covering will touch (remembering that it will shrink and create concave curves). I could use butyrate for the entire project but it will shrink enough to warp the wing. So I'll start with nitrate and switch to butyrate for the final coats when filling the weave. Two coats, with the second going on after the first is dry to the touch. This time, instead of one more coat, sanding, and a final coat of nitrate, I put on two coats of sanding sealer and then sanded. Next time I'll thin the sanding sealer even more, 2;1 or 3:1. There should be enough coats that the wood takes on a sheen. This is important because uncoated balsa sucks up the dope and can cause problem spots when you try to fill the weave. Another thing to note is that the dope melts the fillers, so be careful about repeatedly brushing over the filler. Just get it on, quickly smooth out the brush marks and leave it alone. The Super Fil, however, does not melt and that's why I switched to using it for the fillets.

Then we cut out a sheet of silkspan for the bottom of the wing. The silkspan goes on, smooth side out with the grain running along the longest dimension (i.e. wing spar). The medium grade Brodak silkspan I used before was rather heavy and I finally got some gas model grade from Sig which is much lighter. So this time I'm going with the Sig brand.

I sprayed the tissue down with water, placed it over the wing bottom, and teased out the wrinkles. Whenever the tissue starts to dry out, I wet it down again. Using the nitrate dope, I rub the dope into the tissue along the trailing edge, just on the perimeter. After rubbing it in a bit, it starts to feel gummy and the tissue adheres. Working from the trailing edge to the leading edge to the wingtips, I glue down the tissue, using a single sheet for the bottom. It may be easier to use a separate piece of tissue for each wing tip, but the curve here is easy enough to do it all in one go.

At the tips, I rub the dope in perpendicular to the edge of the wingtip, pulling the tissue down taut. By working back and forth along the wing tips, and reapplying more dope to detach and reglue the tissue, I can work out all the wrinkles (who needs heat shrink). I then do this along the entire perimeter, laying down a second coat.

After two coats along the perimeter, I use light sandpaper to trim the tissue and then tack down the fuzzies with dope. The sandpaper feathers the edge and allows the seam to easily disappear, while making a very clean and close cut. Just need to be careful not to cut through any underlying tissue. Then I flip the wing and cover the top in the same manner as before.

Once the wing has been covered, I go back and put on three more coats on the perimeter. This is important because dope melts dope. When I start filling in the weave of the tissue, I do not want to melt the dope on the perimeter and have the tissue draw back (particularly when I start tautening).

I gave it a final spray down with water and hung to dry. I'll do the same for the control surfaces, fuselage, hatch, servo horns, and the landing gear panels.


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Master member
Thanks for the detailed instructions on how you cover with silkspan tissue. There are many people watching your build that weren't born the last time I covered an airplane using silkspan. I may skip using doculam on the next airplane and use silkspan instead. There's something organically satisfying about the process of covering with silkspan. It takes more time but the results are a work of art.


Skill Collector
Great write up on Silkspan - thank you! I've also got a couple pieces of this from an RC swap meet and am looking forward to giving it a try. And thanks to this, my chances of success are much higher now :D
The time and expense is really the downside. The tissue is cheap, but if you are doing it with dope you need a lot of product. At a minimum, you'd want to buy a quart of butyrate dope (you still need some shrinkage), a quart of thinner, (probably best to buy 2:1 ratio of dope and thinner) and then a pint of your colors. That's easily around $75 to get started. You don't need two kinds of dope, sanding sealer, primer, or silver. You can use clear dope to do a primer coat instead, but it is hard to know when you are cutting clear dope versus tissue. For reference, I used probably 8 ounces of base color for the Eaglet. From my tests with the cub yellow, I'm hoping I won't need as much.

Seems like more people are using Minwax Polycrylic to fill the weave and prime. It's also compatible with a large variety of paints so you can use more economical offerings from the local hardware.


Skill Collector
I think I've got a quart of nitrate dope and a pint of thinner on the shelf now... I'll need to double check and see about the other supplies. I've used Polycrylic on several projects so I know I have that on hand. What process would you recommend using that? Would it still need a dope layer(s) to shrink up the Silkspan?
I think I've got a quart of nitrate dope and a pint of thinner on the shelf now... I'll need to double check and see about the other supplies. I've used Polycrylic on several projects so I know I have that on hand. What process would you recommend using that? Would it still need a dope layer(s) to shrink up the Silkspan?
I haven't used it myself, but my recollection is that you dope down the frame and perimeter of the tissue as above. Then you can switch to the polycrylic to fill the weave and prime. The nitrate won't shrink the covering, but if you are patient and work it while it is wet you can get the wrinkles out and a little tautness. I find the trick is to keep rubbing the edge of the tissue, pulling the wrinkle out, until the dope completely dries. At first, the wrinkles won't come out as you rub it to tighten. Then, when the dope feels like it just dried out, it'll suddenly catch, pull, and stick. I think there is a brief moment when the dope is almost dry enough that it'll hold against the pull if the tissue. That's all I did to get that compound curve, but it would have been easier if I used a separate piece of tissue for each side of the wingtip because I had to pull the wrinkles out down along the wing edge. I was just stubborn and wanted to get it all done using only two sheets.

You can see below that just using water gives enough tautening for visuals. It's not drum tight but it still offers a little stiffness.

Paul Kohlmann wrote some articles about his Miles M30 build that used Polyspan and Polycrylic. You can find the full set in pdf online. The Polyspan shrinks a little under heat, not as much as iron on covering, but enough to get out the wrinkles and work around compound curves.

Edit: Now that I think about it, Paul used Mod Podge to adhere the fabric. So he only used stuff you can get at the hardware store so you don't have to go all in on dope to do fabric and paint.


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Did the ailerons, hatch, and fuselage bottom tonight.

I cut some slits to go around the gear wire. The tissue closes up on itself around the wire well enough. The silkspan is more delicate than the Brodak brand. In particular, Brodak did not have a noticable grain but Sig's does. I find that the weave opened up in a few places as I stretched across the grain. I think it'll fill in with dope, if not, I can put a few shreds of tissue there to plug the hole.


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Right, finished applying the tissue and installed the control horns.

After the five coats of nitrate on the perimeter to glue down the tissue, I applied three coats of nitrate over the rest of the tissue. I generally wait 8 hours between coats for the dope to gas off. This did add a little tautening but did not fully fill the weave as shown.

So I switched to butyrate and did two coats. This made it drum tight, but it started clinging to the underside of the trailing edge and in places on the leading edge. I should have stuck with nitrate only in retrospect. I added one final coat of butyrate that I tinted with black dope. This is so I can tell when I am hitting the dope when sanding down and when I have gone through the top layer.

With the last two coats, the weave had filled and the dope starts to run together and float on top. I brush on enough dope to make a smooth pool of it on the tissue. This will dry out very smooth with no brush marks. It's at this point that I start building up a layer of dope on top of the tissue.

I'll sand it down a bit with 320 tomorrow and start the primer. I'll mainly sand over the wood areas as the dope dries smooth when floated over the open bays.


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The dope over the wood didn't flow out very well and left a very mottled surface. I gave it a very light sanding with 320, but I found that I can't sand it down unaided because there are low spots that will still hit the tissue.

Two coats of 50-50 primer were sprayed on, waiting until the first coat was dry to the touch before adding the follow up coat. The primer will give a contrast to tell me when I start sanding into the tissue and dope. After a day to dry (otherwise it'll sand off gummy), I'll sand it down with 400, stopping as soon as the primer is gone.

I have to be very careful along the edges of any bay as the edge makes a pressure point that I can sand through and cut the tissue in just a couple of strokes. That's why I took great care to round them all off before covering.

The pictures show how the tautening pulled the tissue down onto the trailing edge. It's ok on the top as it clings to the entire surface and I'll smooth it out with primer and sanding. On the bottom, it only catches in small spots that I won't be able to hide unfortunately.


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Right, didn't have time to go at it every day, but I finally got my first round of sanding done. Real happy with how the tinted dope layer worked out. Definitely helped with making sure I wasn't cutting too deep.

Put on another two coats of primer and I'll go at it again in a day or two.


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