SERCEFLYER

I just started trying to dope my silk-covered frame. I sprayed some Sig regular butyrate Supercoat on, expecting the silk to pull tighter. Unfortunately, after drying the silk has sagged and looks terrible.

What did I do wrong?
Is there any way to correct this without re-covering?
It was a little chilly and damp in the garage when I did the doping-- could this have been the cause?

Thanks

hankpajari

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Sorry duplicate post.

hankpajari

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I use Aerogloss butyrate dope when doing silk and dope. I thought Sig was Nitrate dope? Don't know the difference as far as shrinking is concerned.

When I silk (which is very rarely these days) I dope the ribs and leading/trailing edges first and let dry. I wet the silk and get most of the water out. Then I lay the silk on and dope around the edges while pulling it TIGHT. As the silk dries it gets tighter. I then brush on a light coat of dope trying not to get runs under the silk. This will pull the silk tighter and most wrinkles will dissapear. Then a couple more coats of dope, lightly or else you end up with runs under the dope or drips onto the bottom surface. Once sealed completely you can paint.

BTW putting on dope over a wet surface will cause blushing. It will dissapear after another coat.

SERCEFLYER

The Sig dope I used is definitely butyrate-- the regular kind. Sig makes a Lite-coat that shrinks less. Any ideas on how to save this bird?

SF

Jim Thomerson

As said, get the framework well doped before applying silk. If you don't the dope will bleed through the silk and you will have the open bays shiny and the over-the-wood parts still dull. I like to use nitrate, but Sig Supercoat will work fine. Put the silk on sopping wet and take your time to get it on perfect, grain straight down the wing spanwise, no wrinkles, etc. Silk will go around curves very nicely if you are patient. Cut the silk with really sharp razorblades. Stick it down with thinner or thin dope while wet. It will blush but no problem. Your silk should look perfect as soon as the water dries. If not, fix it. Put the first dope coats on very thin, like 70% thinner. Do this until the grain is sealed. This way the runthroughs you will get won't show. After the grain is sealed go with thicker coats.

Sig makes nitrate, Litecoat plasticized butyrate, and Supercoat butyrate which will shrink a lot. I suspect spraying is not as good as brushing. Butyrate keeps on shrinking so your problem may just go away. If it doesn't, I'd suggest brushing on a coat of Supercoat, with some retarder, and seeing what happens over a couple of days.

If you use Aerogloss, I suggest you get some plasticizer from Dave Brown and use as instructed, otherwise you may have real problems with the finish shrinking and cracking. A lot of us have had some real problems that way.

Jim

Gollywock

As hank said, it needs to go on wet. That's what causes most of the shrink. If you are having trouble with the dope blushing (milky looking) either wait for a really dry day or stir some Sig retarder into your dope and brush it on. Jim

SERCEFLYER

Thanks a lot for the advice. This is what I'm puzzled by: why isn't the dope shrinking. I've doped a lot of tissue-covered models and know about over-shrinkage. Do I have a bad batch of dope here?

What exactly will the retarder do? I thought the retarder would halt further shrinkage.

Strat2003

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Where did you get the silk you're using? I've had that experience with some of the cheap silk I've used (the $2.00/yard variety). In MOST cases it tightens up after a few coats and a few days drying time. In a very few cases it never shrunk really tight and remained sorta ripply. It's happened with nitrate and butyrate dopes, both from Randolph.
I've never had this problem with the expensive ($18.00/yard) silk sold strictly for modeling use.

By the way, I always cover wet and keep it wet with a sprayer to give me more time.

SERCEFLYER

Yeah, my silk is the cheap stuff from Thai Silk. This will really be a pain if I have to re-cover the whole thing!

Mike Denest

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After a couple of coats of dope over the framework, apply the silk "wet", but not soaking wet. Carefully pull the silk working out the wrinkles then apply a thin coat of dope to "lock" in the silk, but not too tight so as to induce warps. It's a trial and error thing, once you get the idea it's easy to do. Ok, continue to work around the edge of the silk applying fresh dope and continue to gently work out any wrinkles. Allow to dry for a few hours then come back and trim the excess. I use a 320 grit sandpaper to cut the weave.

As far as "what did I did wrong" is concerned, you need lots of heat. A warm ventilated room or at least a 70 degree sunny day is preferable. Chilly and damp will not work while applying a silk and dope covering job. Wait for a nice, sunny, warm day and try spraying another coat of butyrate and let it sit in the sun for a while. It should tighten up. If it tightens up, then press on. It sounds like you may not have stretched the silk enough. You will see this when you wash down the airplane at the end of the flying session. The covering will go limp then tighten up again. This is ok as over time, the butyrate dope will continue to shrink.

Square Nozzle

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In my experience, it takes more than one coat of dope to shrink the fabric. I have covered "full size" airplanes using the old non shrink fabric and what you are seeing is the norm. It takes several (3 to 4 or more) coats of butyrate dope to shrink the fabric. Keep on dopin! I think you will eventually see shrink. Or you can pull it all off and do the wet silk method as described. That is the typical application method. Wet the bonded edges with thinner and the silk will lift easily.

Mike Denest

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To remove a doped covering, try acetone or MEK. Dope thinner is very expensive to waste!

hankpajari

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Thanks, I forgot about getting the weave right. Glad to know Sig makes butyrate dope, Aerogloss is hard to find here. The thinner is ridiculous! I remember when I was a kid I could go to the airport and buy clear dope in bulk. I think it was about $.50 a quart! OK I'm old.

As far as saving the covering you have already done the guys here have given the best advice, try to soften the dope holding the silk to the airframe and stretching it out again. Maybe with patience it will work. I know you don't want to hear this but the last time I silked a wing (original Jensen Ugly Stick a couple of years ago) I had to pull of two panels and start over.

Good luck, once you get the hang it really does work neat!

HighPlains

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Hey, I used to go to the local airport and buy a quart of clear and quart of thinner for about $.50 too! But the last airplane I silked was a Falcon 56, and that was only 35 years ago.....I started young.

kdheath

FWIW, the class act in model dope is from Brodak. It's made by Randolph to Brodak's specs. They also carry nitrate clear. Dries faster than Sig. Rubs out really nice. DuPone 3608S thinner works well and costs way less than "name brand" thinner.

hankpajari

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Cool, where do you get it? What is the shelf life? I have 20 year old Areogloss dope I still use.

kdheath

www.brodak.com or www.clstunt.com for Brodak dope. Shelf life darn near forever if stored properly. Store the cans upside down in a cool place. 3608S from an auto paint store.

Jim Thomerson

I was looking through some 1960's magazines and found the recommendation to spray the silk, after it is watershrunk, with spray laundry starch to fill the pores. Then dope as usual. Said to work real well. Please try it on your airplane and tell me how it works.

Jim

APope

I came across a very good article on covering with silk in the October, 1993 issue of Model Aviation, in the AMA digital archives. Maybe this link will work for you: http://www.modelaircraft.org/Members...ge_article.htm

Gonnacrash

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SF
Full scale experience in '56. Covering with Irish Linen ( I know its not silk but) couple of the control surfaces were real loose....
Ice pick punches a couple of holes, poured in cup or so of water.... shook it around and around and around... after "washing" down the outside with thinner, threw it out in the sun and BINGO tigher
than a drum in a couple of hours. Just a thought.
Don-Basehor, Ks

laker500

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I had some problems with Sig dope and blushing so I sent an e-mail off to Sig customer services. They were more than helpfull with my problem. They sent me several e-mails over the next couple days and answerd all of my questions. If you are using their dope I'm sure they would tell you anything you needed to know. I learned more from them then I would probally have ever picked up trying to figure it out myself and ruining a couple planes in the process. Jeff

buzzard bait

I know this is an old thread--I found it while looking for a more recent one.

I was wondering what happened--did you get it to shrink, and if so, how?

I've been experimenting with Thai Silks (which is pre-shrunk) and found that Sig Supercoat was just barely adequate if I carefully stretched the silk as much as I could while it was wet.

But the best results came from Randolph Tautening Nitrate--that gave very good shrinkage.

Brodak Nitrate is the worst for shrinkage because it has a lot of plasticizer; that makes it good for other things, but worthless for Thai Silks.

While Mike is right about not pulling too tight with model airplane silk, Thai Silks are different because they shrink so little on their own.

Jim