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Aug 15, 2019, 04:43 PM
I Hate Orange Props
MicroPlane6's Avatar
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Help!

Doping silk


I'm having trouble doping silk on an open frame. I attached the silk be getting it wet, using pins and thinner over pre-applied dope to tack it down, and got it on wrinkle free. But when I applied a layer of dope later, it turned out very bad (see pictures.) I've stripped it off and re-applied silk, but am unsure how to proceed. I want the wings to be left unpainted.
I'll be orienting the wing vertically this time, to prevent the dope from pooling on the other side. What else should I do? Should I be thinning the dope on the first coat? It's Randolph nitrate dope.
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Aug 16, 2019, 08:33 PM
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I always thin full scale airplane dope 50%. Are you using taughting dope? Where did you buy your silk? Many have bought silk from Darma Trading in white and had good luck with it. Two years ago my friend. And I bought colored silk, red and yellow. This colored silk will not shrink with dope. It appears there is something in the silk after dying that reacts with the dope. Neither nitrate or butyrate will shrink it. Whatever is in the colored silk may now be in the white. I never had this problem with silk from SIG, but it is $30 per yard. When I discovered the problem, I made up some test panels to see if I could find a process. Last week I pulled these panels off the shelf, after two years they did not shrink. I was using colored silk to get a transparent colored finish. On previous models I used Koverall and paint. Today, that is all I use.
Aug 16, 2019, 10:41 PM
Registered User
If you use a brush, it is very easy to push the dope through the silk, with the results seen in your pictures. Here's a technique I have used to avoid it:

Make a stack of toilet paper, about twice as long as the chord of the wing, half a dozen layers or so thick. Saturate with dope, use enough dope so that it is pooling on the top. Pull the wet tp over the wing, from root to tip in a slow, smooth, continuous motion and let the surface tension do its magic. Only one swipe over the whole wing surface with saturated toilet paper, then don't touch it until the dope has dried. If there are dry spots, you can repeat the process. It is easier if you put the wing on some stands, so it is sitting above the workbench and tp can be draped over leading and trailing edges while pulling. If the wing is large, have an assistant ready to add dope on the tp during the pull to keep it fully saturated, but don't let this interrupt the motion.
Aug 17, 2019, 12:22 PM
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mhodgson's Avatar
Practice on some test frames first. Once the silk is on apply some 50/50 thinners/dope around the edges to make sure it is stuck on well. Then dope wing using whatever method with thinned dope. Keep applying until happy.
Are you using natural silk or a cheaper man made silk?
It is a skill to learn a process that works for you.
Aug 17, 2019, 02:02 PM
Registered User
Silk has such an open weave, it's easy for the first coats to drip thru. Year back I learned to use a "brush" made from folded silk or other soft material, The trick is to pull awet loaded brush across the silk, dragging the caot from the brush On to" the silk's surface. Work from wet to dry and NEVER go back until it is dry. Some have reported success using a cheap wide foam brush in same manner. the point is not to push dope thru t esilk, either forming a blob just insid e the weave, or just as bad, have tha tblob drip onto backside of surface opposite. Blobs and drips make unsightly blemishes!

AFAIK, the ultimate weight saving way to get a sealed dope finish on silk is to mix one packet of Knoxx gelatin powder and dilute it about a cup of very hot water. We're looking for a sticky clear liquid, about the consistency of light cream. Applied with a foam or silk type brush, in same manner mentioned,. Allow that coat to dry, looking for pin holes and open weave in a strong light. R-ecoat as necessary. When satisfied the weave is sufficiently sealed ,THEN begin brush coats of 50%/50%dope thinner (with plastisizer if desired). But do not over brush! Build to finish desired. Dried gelatin has no perceptible weight!!! This technique has been successfully used on small rubber models, weighing no more than those normally covered in tissue, with HUGE increase in covering durability. I've often wondered if spray -can laundry starch might be used in same manner?

BTW- I'm of the opinion that all shrinkage should occur at the wet shrink stage (either by covering with wet silk or water shrinking after dry covering) Any contraction after that initial shrink is probably going to warp or damage your structure. Dope to seal, not to shrink!
Aug 17, 2019, 02:44 PM
Registered User
I certainly agree that the "sealing" coats of clear dope on the fabric should be heavily thinned...… so that the dope wets the fibers but the method of application nevertheless has to avoid pooling of dope on the bottom side of the fabric.

I have used soft brushes barely wet/damp (with water based poly) on thin sheet balsa - applying several coats, with each coat drying within 10 minutes or so - as way of sealing the thin balsa without deforming the thin sheeting because of the water base.
I think the same "spit" coats of thinned dope could be used to seal the silk so that later on heavier/wet coats could be applied, without them leaking thru the weave.
I imagine this is what "jkettu" and the first part of "packardpursuit's" posts above are accomplishing.

Here's another approach I just read in the instructions of a Royal kit of Bleriot model (found on outerzone).... applying the sealing coat of dope on wet fabric; what do people think of this idea?

'... to apply the covering, cut it oversize, hold in place and spray with a fine, fine mist of water sprayer carefully working out all the wrinkles and dope it while still wet with the brush nearly parallel to the surface and lightly, lightly stroking it. If done while wet, the dope will "float" on the damp surface and dry "white or cloudy" but will require far fewer coats and the cloudy look will disappear after the 2nd or 3rd coat.....'

the knox gelatin method is also great

Michael in Ontario, Canada
Last edited by 2michaely; Aug 17, 2019 at 02:58 PM.
Aug 18, 2019, 06:21 PM
Registered User
Yes, you can apply dope over a wet surface. It will turn white, it is called blushing. Additional coats usually clear this up, depending on the weather.

There a many ways to prevent pooling of the first coats of dope, many have been suggested. I have heard of, but not tried, using a foam brush. Working small areas and turning the panel over to use gravity to prevent pooling works for me.
Aug 23, 2019, 08:47 PM
I Hate Orange Props
MicroPlane6's Avatar
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I've coated the wings with a gelatin mixture. We'll see how it works out.
Last edited by MicroPlane6; Aug 23, 2019 at 11:17 PM. Reason: Spelling
Aug 26, 2019, 02:47 AM
I Hate Orange Props
MicroPlane6's Avatar
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It worked well. I added a light coat of white paint. In some areas, I didn't use enough thinner, bit it mostly turned out well.
Aug 26, 2019, 11:35 AM
Registered User
Quote:
Originally Posted by MicroPlane6
It worked well. I added a light coat of white paint. In some areas, I didn't use enough thinner, bit it mostly turned out well.
Thanks for the followup information on how the paint worked over the gelatin filler..... I'll likely try this as well.

Michael in Ontario, Canada


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