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Building Tips and technical articles. => Paint and finishing => Topic started by: Martin Gallant on October 23, 2010, 01:11:54 PM
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Hello. This is my first attempt at silk and dope finishing.
I am using 5mm Habotai from Dharma, and Randolph nitrate dope and thinner.
Silk was applied wet and was wrinkle-free after doping the perimeters and allowed to dry.
I did a good job prepping the wood, and I know the silk is not pulling free from the edges.
As soon as I put down the first coat of dope on the open structures, the fabric sagged on me.
I am now at 6 coats (Thinned roughly 50%) and have left the structure alone for 3 days.
The sags and wrinkles seem to be pulling out with time, but I'm not sure how to proceed.
Should I leave it alone and let it cure longer?
Would brushing the open structure with pure thinner help (I'm thinking this would reflow the dope solids)
Should I just keep adding more coats at 50% until these go away? (I suspect it might just make it worse)
On my test piece a month ago, the fabric sagged the same way on the first coat, but pulled back nice and tight after 5-6 coats and letting it sit overnight.
What normally causes this to happen?
I'm thinking my first coat was a little too thick (not thinned down enough) and the sags are where heavy dope puddled under the fabric.
Thanks in advance guys. I really like the appearance of these finishes and want to learn how to do it right.
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I do silk & dope all the time.
Just like you, expect I use butyrate dope. It seems kind of sagged until I get up to 6 or 7 coats, then ti tightens OK.
Your procedure seems OK. Remember, the silk doesn't shrink, just the dope. So if it was wrinkle-free to begin with, is should go back to wrinkle-free when the dope shrinks.
I've never been a fan of nitrate. I need all the "fuel proof" I can get. Maybe "fuel-resistant" would be more honest.
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Just a suggestion, I use silk over mylar, the very thin type used by the freeflight competition guys. You heat shrink the mylar and then put the silk on top. No sagging and no need for lots of dope coats (the covering is airtight). The big advantage is that the resultant covering is lighter than the conventional silk covering.
The drawback is that a lot of folk find it difficult to do the mylar covering, I don't know why! I don't aspire to the elite finishings though.
Regards,
Andrew.
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So do I wait for the dope to cure some more, or put more on? The weave looks completely filled to me already.
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I haven't used that kind of silk. Not a problem with Esaki type silk. There was a suggestion that it seals so well with dope that thinner fumes are trapped inside the structure and take a while to dissipate. I make my hollow structures so that air can get in and out, as when an airplane is sitting out in the sun and heating up.
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Mike,
I have had similar problems with nitrate over silk (I only use Sig Supercoat clear buty now which is taunting). If you used the non-taunting type, it will take time to finally pull tight. I would let it gas off for a week and if still wrinkled, then go with a coat or two of taunting buty. You will have to eventully go to buty anyway, so might as well do it now. I tried the thinner and heat technique with little success. Only thing that really worked was the taunting dope. As said eariler, it's only the dope that really shrinks not the silk.
Jim
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The way I put silk on is to spray it with water mist from a spray bottle, then let it grow, yes when any fabric gets wet it grows, so this is where you take your time and leave it for a few minutes while making sure that the wrinkles are pulled out, then while it is still damp tack the edges of the silk all the way around the frame.
Once the silk has dried it will go back to the original state and take up the slack and be tight.
If you have wrinkles butrate tautening dope over the nitrate will take up the rest of the small wrinkles, remember after the last coat, especially on structures that warp easily, when the dope is touch dry but still has a smell, pin the structure down, with small spacers between the structure and the building board to let air flow through and then let it stay there for 3 days, that way the dope sets with the wing in a warp free state.
Lionel.
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Just a suggestion, I use silk over mylar, the very thin type used by the freeflight competition guys. You heat shrink the mylar and then put the silk on top. No sagging and no need for lots of dope coats (the covering is airtight). The big advantage is that the resultant covering is lighter than the conventional silk covering.
The drawback is that a lot of folk find it difficult to do the mylar covering, I don't know why! I don't aspire to the elite finishings though.
Regards,
Andrew.
Andrew,
Do you use dope to apply the silk to the mylar, or is another adhesive required for adhesion?
Thanks.
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Hello. This is my first attempt at silk and dope finishing.
I am using 5mm Habotai from Dharma, and Randolph nitrate dope and thinner.
Silk was applied wet and was wrinkle-free after doping the perimeters and allowed to dry.
I did a good job prepping the wood, and I know the silk is not pulling free from the edges.
As soon as I put down the first coat of dope on the open structures, the fabric sagged on me.
I am now at 6 coats (Thinned roughly 50%) and have left the structure alone for 3 days.
The sags and wrinkles seem to be pulling out with time, but I'm not sure how to proceed.
Should I leave it alone and let it cure longer?
Would brushing the open structure with pure thinner help (I'm thinking this would reflow the dope solids)
Should I just keep adding more coats at 50% until these go away? (I suspect it might just make it worse)
On my test piece a month ago, the fabric sagged the same way on the first coat, but pulled back nice and tight after 5-6 coats and letting it sit overnight.
What normally causes this to happen?
I'm thinking my first coat was a little too thick (not thinned down enough) and the sags are where heavy dope puddled under the fabric.
Thanks in advance guys. I really like the appearance of these finishes and want to learn how to do it right.
Hi Martin,
Whether you use nitrate or butyrate dope, for the first two or three coats after the silk is dry (from the water you sprayed on it prior to applying it to the structure) you should be using tautening type dope. The first coat or two will penetrate the weave and encapsulate the silk, then when the dope shrinks it tautens the silk. After the first two or three coats of tautening type dope begin using non-tautening type dope, whether it is clear or colored.
The silk, as you have covered and doped it, may shrink over time. I'm not sure whether you will have a great degree of success with the (apparently) non-tautening nitrate used. Tautening type nitrate shrinks a little more than tautening type butyrate, however the film tensile strength of dry nitrate is less than that of dry butyrate. The greater degree of shrinking of nitrate coupled with a lower finished film tensile strength can lead to cracking of the finish as the dry nitrate shrinks beyond its tensile strength.
I use butyrate type dope from the bare wood on out to the final coats of clear on models; I have found no advantage to using nitrate. The only reason that I ever use nitrate is when covering full-scale airplanes and the Supplemental Type Certificate requires its use.
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I'm using Randolph 210 which is labeled as taughtening.
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I've never had your problem. If all the pinholes are filled I'd go to thicker dope for the next couple of coats, maybe 80% dope, 20% thinner. I take it you are bushing the dope on.
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I also use Dharma silk and haven't had any problems, I use the lighter (cheaper) stuff. Randilph makes Butyrate in both tautening and non. Their colors are all non tautening, called Randolp directly about the colors. I apply the silk dry, applying dope only around the edges to get it to adhere. After I get all the wrinkles out, I spray the silk with water and after it drys, start putting thinned nitrate over it until filled and before applying the color, two coats of butyrate clear.
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I love using silk and been using it since the late sixties. Never had an issue until 2005. Turns out it was
"bad" silk. I found an old bag of silk, used it with no problems. These days, I use polyspan. Not my
favorite, but after my results with the last batch of "new" silk, I just don't trust what I'm
getting.
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Hi Kim,
Very late in the day, I noticed your question. I use the dope to stick the silk to the mylar. I use thinned down contact adhesive to stick the mylar to the structure. Paint it on and allow to dry. Ironing on the mylar activates the adhesive.
Sorry for the huge delay in responding.
Regards,
Andrew.
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No problem! Thanks, Andrew.
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Just curious, after we drape the wet covering (silk pr silkspan) over the structure, we ARE pulling it tight while doping it down, yes?? You can never get it too tight as long as both sides dry at the same rate and time.
Big Bear
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Like silkspan. Silk has a grain. warp and weave. You want grain to go across the wing from wingtip to wingtip.
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I put silk on sopping wet. So the structure has to be well doped. It should be anyway, or the dope will go through the silk, and the open bays will be shiny while the silk grain still shows over wood. With the silk really wet, it sticks to the wood and will stay in place once you get it pulled tight. Then dope around the edges. Cover both sides of the wing, keeping all silk wet the whole time. Then hang the wing vertically to dry, so that the top does not dry faster than the bottom and cause a warp. H^^
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I have covered a couple more structures, and have a pretty good idea where I went wrong since my original post.
1. I did not stretch the wet silk enough when I put it down on the structure
2. The underlying structure was weak, and pulled in on the edges when the dope shrunk
3. The weave was way misaligned and full of waves, further aggravating (2) and inducing a nasty warp
Paying a little more attention to (1) is all it took to fix this
My new wing looks really nice. It is tight and solid, with no warps or buckles.
I'm happy I went through this learning curve. I love the appearance of these finishes. It's worth the effort.
Many thanks to those that replied with their suggestions.
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I have been migrating to silk for the past 4 airplanes. I currently am using 5mm but will order at least 8mm the next time. The biggest reason for that change is that it now takes me 8 to 10 coats to fill and get the silk tight. I hope to improve that with thicker silk.
I put the silk on wet and pull it tight. While drying, I use a hair dryer to dry and shrink it. That produces a reasonably tight, nice looking cover. As soon as I put dope on however, it sags and gets wrinkles. It takes 8 to 10 coats to get back to where I was. But I love the look and strength of the silk.
I use Randolph tautening dope until the color for the last few coats. I have had the dope "warp" the wing but some wet and microwaved towels draped over a straightened and secured "in place" wing, fixes the warp and I have had no trouble with it warping back to the "undesirable."
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I no longer use a hair dryer to dry the silk, as I have had poor results. The dryer heats the air inside the wing. the inside air expands, pushes the silk out, and does not let it shrink as much as letting it dry naturally.
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I read all these posts with a great deal of interest and have a few questions, firstly how much heavier is silk than either silkspan, polyspan or plastic film such as monocote which I don't like using due to the amount of heat needed to do a good tight finish that doesn't get loose in the sun mine always seems to especially the ARF Nobler I have, or the film Tower sells which is cheaper than monokote anyway not yet tried it but I have some I will use on a Peacemaker I am building, secondly where do you get good quality silk, I have used nylon bought from hobby stores by the sq yard years ago and it seems it is no longer available, thirdly when I used nylon I only had to put on 5 coats of 50/50 dope and a sixth coat of 75% thinner 25% dope even less coats if I was going to coat with colored dope.
My main interest is where do you get the silk, I have been to Bangkok a couple of times but because I didn't know what to get I didn't buy any it seems there is alot of very different grades, weaves and weight to silk if I know what I am looking for when I next go back with a sample of silk it is so cheap there, the first time I went there I bought a complete bolt of patterned white silk to use as window treatment for about what I paid for a nights stay in the hotel I was staying in.
So could someone please tell me where to buy silk from and what weight and weave etc., to buy for .15cu.in. through .46cu.in. models, I have a Strega kit but am still debating whether to build it or not I have a feeling the pull of a S.T. V60 will be too much for my healing shoulder.
Thanks Y'all
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Some silks seem to only shrink once :o An old modelling friend of mine dyed some Chinese silk with Dylon, then found it wouldn't water shrink again!! Not sure if it was his fault or the silk? ??? I've only used the Chinese stuff as it comes, a quick run over with a warm iron, lay the cut out piece on a well doped (sanding sealer actually) airframe, spray (mist) with water leave it for a few minutes to soak up the water and expand, pull it tight and run thinner through and on to the doped framework, never fails ;D
Cheers
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I've just found an advert for esaki 5mm habotia silk, and it states that it is not pre-shrunk! Maybe it does only shrink once? ???
Cheers
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I have heard that Dharma and Thai silk are both preshrunk. You can call them and ask. Esaki is not preshrunk and thus IMO easier to use but expensive.
Paul
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Like silkspan. Silk has a grain. warp and weave. You want grain to go across the wing from wingtip to wingtip.
Does the "grain" of the silk run across the bolt, or with the bolt? I've looked hard, and can't determine which way it runs.
Brian
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I apply mine with the grain going along the bolt length. In other words, the finished edges are at the LE/TE before triming the excess silk. H^^
Thanks.
Brian