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May 11, 2019, 08:29 AM
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Silkspan - Tiny Holes


I'm scratch building a Thermic 72 sailplane. I've covered the wing with silkspan to which I applied five coats of 50/50 clear nitrate dope. The surface looked great. I then decided to paint the wing with rattle can rustoleum semi-gloss white enamel paint. At first glance, things looked fine but upon closer inspection I can see lots of tiny holes (smaller than a pinprick) esp in the areas where the silkspan was stretched (image attached). I tested putting on a second coat in a small area but the holes remain. Any suggestions on how best to make the holes disappear?
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May 11, 2019, 03:25 PM
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kenh3497's Avatar
Even though you had/have 5 coats of dope on the surface is still not completely sealed. Maybe the last couple coats should have been thinned less or some talc added to help seal the surface. My guess is the grain of the silkspan was rather course.

Ken
May 11, 2019, 05:19 PM
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Yes, talcum powder (talc) mixed in with the dope. The mixture can even be sprayed.
May 11, 2019, 06:59 PM
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I have a lot of building experience with silkspan, fabric ,etc using dope. Agree that the silkspan was not filled. Big big mistake using enamel over dope especially on fabric. Enamel is not flexible unless you use a flex agent, also enamel is very heavy compared to dope. I really do not see a fix unless you just keep spraying enamel but you probably will be to heavy. Strip the aircraft, the silkspan will realease with thinner and start over. How about a Iight weight iron on. Also you can use butyrate dope non taunting and yes use talcum powder. Add some aluminum paste to the clear and this will help you see areas that needs more fill or just use rando fill, Randolph product.
Michael
May 11, 2019, 11:23 PM
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Looks like issue is most prevalent over the ribs. If the ribs were not sealed before the covering applied, they sucked up the dope you put on the covering hence the pen holes over the ribs. I had cap strips on mine and it looked worse. I sanded and applied a few coats just over the ribs. BTW, I had used talc. Turned out fines.

John
May 12, 2019, 12:13 PM
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Quote:
Originally Posted by Rubbernecker
Looks like issue is most prevalent over the ribs. If the ribs were not sealed before the covering applied, they sucked up the dope you put on the covering hence the pen holes over the ribs. I had cap strips on mine and it looked worse. I sanded and applied a few coats just over the ribs. BTW, I had used talc. Turned out fines.

John

Ruddernecker,

Just out of curiosity, could you confirm John's suspicion... not sealing the side grain on the ribs while sufficiently sealing the wing trailing edges. Typically one seals the balsa over which silkspan will added until it is slightly shiny.


And remember, this is a lightweight, non-scale 72" span Thermic glider, not a scale contest entry. Apply the 10 foot rule; if it looks fine from this distance, just fly it as it and use this a learning experience for the future.


Michael in Ontario, Canada
May 15, 2019, 12:34 AM
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Not sure how to respond since I am John as well as Rubbernecker! What I found was that open silk seals with less coats than silk over unsealed balsa (as in capstrips). I discovered this on a control line Goldberg Buster when I sprayed the silver base coat. There were all these little pinholes only along the capstrips. The open areas looked fine. I lightly sanded all the capstrips and applied a few coats of dope to them. Re-sanded (with steel wool to prevent cutting thru silk) and applied another coat of silver and everything was fine. Keep in mind, clear dope is quite light.

John
May 27, 2019, 05:20 AM
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Unless you want to start over and strip off the old covering you won't be able to put any dope down over the rustoleum. One way to fix the pinholes is to brush more rustoleum into the holes. You can force the paint in a little. However; it isn't going to look too good. You'll have to sand the paint after hand brushing then spray another coat on top of it. You'll add some weight doing this.

I would probably just live with it myself.

carl
Jul 07, 2019, 09:06 PM
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Quote:
Originally Posted by Rubbernecker
Looks like issue is most prevalent over the ribs. If the ribs were not sealed before the covering applied, they sucked up the dope you put on the covering hence the pen holes over the ribs. I had cap strips on mine and it looked worse. I sanded and applied a few coats just over the ribs. BTW, I had used talc. Turned out fines.

John
It's always a good idea to brush on ten coats of dope on all the edges of a silkspan covered open bay wing: ribs, cap strips, and along the sheeting where it ends and open bay begins. Go and buy Striping brush for this. It has long hairs on it and the paint will flow off it so nice you won't believe it. This adds alot of strength but not a whole lotta weight. It's always the edges that need extra coats and not the center of the open bay. Let the enamel fully cure then spray some grey auto primer on it sanding between coats. That should take care of the pin holes. After that spray your enamel. This is the problem with mixing different paint. I use dope from start to finish, but everyone has their own system.
Jul 09, 2019, 06:28 AM
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When I was still painting planes several years back..(probably more than 30 yrs or more ago...) covered with silkspan, I used to spray water to create a mist then brush on clear nitrate dope. When the water mist dries up, it will result in a thin film and cover those tiny holes. Repeat the process if necessary to cover all those holes. Tedious process though but works for me.


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