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Post  Cribbs74 on Wed Nov 21, 2012 8:50 am

All,

I am thinking of doing my Yak using traditional covering methods.

Where do I start? What items will I need?

I am used to doing iron on covering so this is all new to me.

Ron
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Post  JPvelo on Wed Nov 21, 2012 9:22 am

Do a YouTube search for "silkspan" and some great stuff comes up.
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Post  SuperDave on Wed Nov 21, 2012 9:58 am

Ron:

Brodak sells two sheets of medium silkspan for $5.36. Unless they are "out of stock" both light an heavy silkspan are available too.

You'll also need buyrate dope, thinner and retarder. Out of the can Brodak butyrate is INTENDED to be thinned at a ratio of at least two to one. ("retarder" slows the drying rate according to climatic conditions like temperature and humidity.

Modern butyrate is NOT the Aero-gloss of the past. Using it will be yet another "learning experience" for you.

lol!
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Post  andrew on Wed Nov 21, 2012 10:01 am

Ron --

This is a good start: http://winshiprc.tripod.com/covering_with_silkspan_by_jim_ryan.htm

But, a Google search on: covering silkspan will get a lot of hits, but narrative and video.

Read thru some of the articles and if you have any questions at all, there are a number of us old f**ts that have covered with silk, silkspan and tissue. In all likelihood you'll want to use either medium or light silkspan; tissue is great for lightweight coverings, but is more puncture prone and probably not the best choice of a C/L plane.

Try Penn Valley Hobby Center for silkspan; they're quite a bit less expensive.

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Post  SuperDave on Wed Nov 21, 2012 10:10 am

Ron:

Be cautioned.

Most vendors that offer silkspan want to sell it in QUANITIES far greater then you'll need for you YAK.
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Post  Cribbs74 on Wed Nov 21, 2012 10:19 am

Wow guys!

Thanks. So there is a difference between silkspan an tissue then. In my kit it refers to silkspan yet tissue is supplied. I have heard the terms used interchangeably before.

What is blushing? I have access to Aerogloss is it still being made?

Ron

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Post  andrew on Wed Nov 21, 2012 10:56 am

cribbs74 wrote: What is blushing? I have access to Aerogloss is it still being made?


Blushing occurs when the covering is doped under high humidity conditions. Because the volatiles in dope gas off quickly and the surface seals over, water vapor trapped in the dope causes it to take on a whitish color, i.e., blush. This is only an aggrevation, not a major problem. On a very dry day, the dope can be sprayed with thinner or a second coat applied and the blush will go away. You can also add a retarder or plasticizer --- this slows the drying and gives more time for the water vapor to evaporate. Believe it or not (in the olden days silent ), we actually used a drop or two of castor oil as plasticizer.

Dope is an interesting product in that thinner or a second coat essentially re-dissolves the layer. Unlike latex or enamel which hardens and does not return to suspension, succeeding layers of dope literally melt themselves into the underlying layers. Another characteristic of dope is that it continues to shrink over its lifetime. Both butyrate and nitrate are available in non-tautening and high shrink versions. Use high shrink over open areas and on covering that needs to tighten up. Do not use high shrink over sheet balsa --- the shrinkage is so strong that it can warp or bow sheeted areas. Dope will also lift when painted on 90 degree joints such as where the rudder meets the fuselage due to the shrinkage factor. The bridged dope looks like a piece of triangular stock is underneath --- running your finger along the bridge will cause it to crack.

You can apply butyrate over nitrate, but not the other way around. Many folks will recommend using nitrate for surface prep and adhereing the covering --- it does have better adhesion properties than butyrate. However, I've covered a lot of planes with tissue, silkspan and silk using only butyrate with no problems of lifting. Butyrate is fuel resistant; nitrate is not. If prepped with nitrate, you will still need to surface seal with butyrate.

Good surface preparation is the key to a good covering. We used to mix talcum powder with clear dope to use as a filler; it's heavy and there are better sealer/fillers available.

Generally, I would apply a coat of clear low-shrink over all the wood surfaces. The wood will feel like fine sandpaper because the dope lifts the grain and makes it stiff. Sand with 220, then 400. Apply another coat with talc. Sand smooth again. Continue until the surface feels like glass. When sanding, take the filler all the way down to wood --- this allows the grain to be filled, but holds the weight down. Some folks use a lacquer based sanding sealer or automotive primer. I have not and don't know how well the butyrate will stick. Cover and apply color and trim.

For brushing, thin with about 30 to 35% thinner --- this allows the brush marks to flow out before the dope thickens. For spraying, you can thin around 50%.


Last edited by andrew on Wed Nov 21, 2012 11:20 am; edited 2 times in total
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Post  SuperDave on Wed Nov 21, 2012 11:08 am

Ron:

Clear Aero-gloss and thinner ARE still being made by Midwest but colors aren't.

The former Aero-gloss plant was destroyed in a fire a number of years ago and the owners declined to rebuild due to a declining market for their products.

BTW: silkspan and tissue are of the same composition.
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Post  Ken Cook on Wed Nov 21, 2012 2:59 pm

For someone learning to do this the first time, I highly recommend Sig. Sig is more formulated for brushing in my opinon.The problem with all dope finishes is the cost nowadays. Sig dope uses a slower thinner and the frequency of blushing to occur is only going to happen if your painting in the rain. This won't be the case with Brodak. Mix the clear with about 40% thinner this is easier to guesstimate using a 8oz. jar. A soft squirrel hair brush 1" wide and start doping the perimeter framework . This typically require a minimum of 3 coats. The silkspan to be used should be at least a medium and I would recommend K&S or Sig. Sig dope is more fuel proof than Brodak dope and Sig Litecoat probably provides the clearest of clear when it comes to dope available. Brodak now currently makes a Crystal Clear which is quite nice but it does carry a high price tag. Sig Litecoat as a finish coat also has the lowest shrink rate of any of the dopes. Even though Randolph makes Sig, the formulas aren't the same and I've seen Randolph crush the framework as it was really designed for commercial aircraft. Sig clear runs about $30.00 a qt and the Thinner is about $16 a qt.. Even though this material is cut, it really doesn't go far. The thinner is really consumed due to the material evaporating as well.

Sig makes dope in nitrate and butyrate. Butyrate is the fuel proof version, however, I use the nitrate for base coats for several reasons. It dries quicker, it also builds faster, and nitrate is a universal base for almost every known finish out there. Sig nitrate can be thinned with butyrate thinner. This isn't the case with Brodak and you need to purchase 2 types of thinner. Butyrate can be used from start to finish though. Nitrate can be applied first then top coated with butyrate. Nitrate CAN NOT be applied over butyrate. Most hobbyists shy away from nitrate using butyrate all the way to finish coats.

After I apply the thinned dope to the wing, I cut a piece of silkspan. I wet the silkspan and lay it on a towel to soak up residual water and this usually leaves the silkspan wet enough to cling but not soaking wet. I lay it on the wing and begin to brush dope over it. I keep the dope rather thin as it will soak through the silkspan and soften the underlying coats. Some people and I even have done this is brush straight thinner through the silkspan. I usually only do this when applying tissue. After the attachment coats, it usually takes 3-5 coats of clear dope prior to the color. Colors are heavy, and they need to be used sparingly. So if you haven't already done this, be prepared for a big expense, and a covering that's very susceptible to damage. The trade off is, it's lightweight when done correctly, it also strengthens the wing and stays tight over the course of time. The downside is, sticks or even a landing on high grass can tear it open. I use Dave Brown Flex All which is a plasticizer that makes the silkspan and dope flexible like Monokote.

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Here is some pictures of my Brodak Oriental. This was the first rubbed finish I ever did. This is Brodak Insignia white and Piper Metallic Green with a Sig Litecoat topcoat. Now, most will suggest using the same brand products from start to finish. This kind of information I would certainly forward onto someone new who's never done this to eliminate problems. I've had a lot of doping experience and made many trials with thinners and other brand top coats. All this work isn't required for a fuel proof finish, it just shows the capability of the products being used. These finishes take a lot of time and patience. After my final coats of clear were shot, the plane sat for nearly 3 weeks until dried enough then it was wet sanded and polished. That part took nearly a week in itself to polish. As much as I love doing a polished finish, one rip and you want to flip out. Ken
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Post  SuperDave on Wed Nov 21, 2012 4:45 pm

Ken gave you a good synopsis of the process of using dope (as a FINISH that is) Wink

Now you better understand why synthetic coverings evolved so successfully.

A cost comparison between the two will make your jaw drop. affraid
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