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Silk Covering

This subject begins with an email message to SAMTalk, the Free Flight Mailing List, and the Phoenix [AZ] Model Airplane Club list. Here’s what I wrote: [all italicized notes are from me]

"Next project here will have silk covering, but it's been so long since I've used it, thot I'd ask for some thoughts on the matter. [My last silk covering might have been a $1.00 silk scarf covered cl stunt model a loooong time ago!]

To prevent the bleed through of early coats of dope, there are at least 3 methods that I've heard of to help fill the pores:

-use a foam brush

-use an overlapping layer of toilet tissue to drag along the surface

-use dissolved gelatine as first coat [what does this do to transparency or appearance - and how is the gelatine mixed?].

Anyone care to support any/all of these? Any other approaches?

BTW: I have some of the Starline/FAI dye to use in sprayed clear nitrate - what's a good starting point for how much to use?"

AL, A. A. Lidberg model plan service, www.aalmps.com

I haven’t even begun cutting wood yet, so I can’t tell you what worked for me. When the model is done, I’ll pass along my own experiences. Here are the responses.

Graham Knight in London, England wrote

Don't use toilet paper for the Meniscus method Al, not unless your US toilet paper is very different to our stuff! Toilet paper will disintegrate and leave crud all over your nice silk, use a piece of heavyweight covering tissue instead.

Of course you could use the best method of all and cover silk over Mylar!

Zero bleed through, quicker, and the end result is usually lighter than silk/dope alone.

 

Thomas Ryan, in an executive jet, somewhere over the US, wrote

Heard bad things about gelatin method. Gets messy when damp.

I'm most impressed with light silk over polyspan. Requires about as much dope as silk alone and is still translucent. Looks same as models covered with silk over silkspan technique but lighter and VERY strong.

 

Hank Baer, [where?], wrote:

I've been using the so called "toilet paper" method of applying dope to silk to fill the pores for many years with great results--After you get the hang of it, you can actually fill ALL the pores in one coat with no drip through--One hint:...trim off the trailing edge of the toilet paper in a straight cut with a pair of scissors--The little bumps from the perforations will leave tracks if you don't and will show in later coats--I use a 60% dope/40% thinner mix to do this and a soft 1 inch brush to puddle the dope on the paper.

 

Hank Sperzel in Omaha wrote:

Finishing Silk or What I did on my summer vacation

Hi Al, the best method I've found is many, many coats of very thin dope sprayed on. Thin the dope to about 60/40, that 60% thinner and 40 clear dope. It'll take about 4 or 5 coats before you see anything. The dye, is it Higgins ink? Sal Fruciano (Starline) said one part dye, 10 parts thinner, and on part clear dope. That is what I've been use and it comes out OK. BTW spray the dye on and then put two or three coats of clear on top. You got'ta spray the stuff on other wise it looks like crap. BTW In my option there is only one use for toilet paper and it isn't to wipe paint on with.

[I doubt that Starline dye is ink – inks most often seem to have a water base, so how could that mix with dope or thinner without making a mess? Anyhow, see my note at the bottom about use of ink to color clear Micafilm]

 

Harry Cook, [where?], wrote:

[relating to the ‘toilet paper falls apart’ comment]

That is so true Graham--year ago was painting a McCoy 49 Black case and kept getting a

"mottled" paint job using a spray can --finally figured out it was the restaurant

napkins used as final alcohol wipe down that was leaving tiny paper bits on the parts

--about drove me nuts as at first blamed the spray paint --use a t-shirt.

[I wrote about using a special anti-dust cloth called the "Cadie Dust Catcher" when vacuforming after reading this one - see cadiecloth.gif .]

 

Floyd [who/where?] wrote:

Al. Silk has been my choice for almost 50 years. It has taken a lot of experiments to make it look good. The main trick is to use dry coats initially; (not too much thinner) otherwise, the dope will soak through and pool on the back side. Unlike tissue, which soaks up the dope and spreads, silk needs several dry coats with a brush before the weave will seal. It is a lot of extra work, but it's worth it!

 

Ed Piggott, of Phoenix, AZ MAC wrote

I have not covered anything with silk since I was about 12 years old. Once was enough. It was a total disaster. I offer up this episode to show what not to do. Although, you probably know about such a mistake as this already, and how to avoid it. I had finished building a large four foot span towline stand-

off scale glider that bore a strong resemblance to a man carrying sail- plane of the 1930's. The name of the kit escapes me, but it was complete with a scale pilot and instrument panel. I was quite proud of it.

It was ready for covering and doping and so, I bought some red silk (for covering planes) at the local hobby shop. I attached the silk to the fuselage, wings, etc. and then made the mistake of spraying the covering with water from an atomizer to shrink the covering tight. There were no instructions with the package of silk so I didn't know what was about to happen.

When the silk dried, I had a pretzel shaped like a sailplane. The whole thing had to go into the garbage, as I was not successful in getting the silk off without damaging the framework beyond repair. If I had an older person who knew better as to what to do in the first place to steer me in the right direction ------ oh well.

 

Don Blackburn in Texas wrote:

Guys, don.t laugh but I read this tip in Air Trails maybe 30 years ago. It works. After you get the silk applied and dry, brush on a coay of milk. Seems to fill the pores of the silk and eliminates 2 or 3 coats of dope.Back then we used whole milk as 2% had not come out yet. Just think of the milk as really thin white glue.

 

JIMSAM40 [who/where?] wrote

DON A THIN MIXTURE OF CORNSTARCH WORKS WELL ALSO, I IMAGINE FLOUR WOULD ALSO WORK AS LONG AS IT WAS THIN

 

Floyd wrote again:

Don. The -problem I have is with applying silk dry. I always apply silk wet. I wet the silk and wait for it to absorb all the water. Then stretch it onto the structure and pull out the wrinkles. A coat of dope around the edges will nail it into place. When the silk dries and shrinks, it will be ready for your doping.

It is necessary to prime the balsa structure with several coats of clear dope so that the silk will have something to hang onto.

[On priming the wood – I use a 50/50 mix of nitrate clear and Ambroid or SIGment to produce a good base for covering. With a couple of coats of the mix, one can use thinner on a brush to activate the sticky qualities.]

 

Don Blackburn wrote again:

Floyd, I did'nt mean to imply to apply the silk dry. I put it on wet also, secure it with dope, then when it is dry brush on a coat of milk. When that is dry, go on with the usual doping.

 

Jack Jellá in Salinas, California, wrote

Hi Al, Seems everyone has a preferred method for covering with silk, and I assume that all must provide a satisfactory finish. But, I would like to suggest another method that I have been using for the last 25 years with excellent results. I have read of all manor of materials used to seal the weave and prevent the dreaded dope "run through" that spoils so many otherwise beautiful covering jobs. Forget the toilet tissue, foam brushes, milk, jello or whatever else you've been using and let's use the one material that is completely compatible with the silk, the dope.

I pre-dope the silk with one coat of very thin nitrate before I begin to cover. Here's how I do it. I begin by constructing a frame of 1/4" balsa or pine strips slightly larger (an inch or so all around) than the structure that I want to cover. I use my small 4" disc sander to cut the end of each piece at a 45 deg. angle (like a picture frame) for a better glue joint, medium CA works well and speeds up the process. Except for very small frames several spacers placed across the frame will add rigidity and maintain the proper width. Glue them on top of the frame so they won't touch the silk, they also make convenient handles when you are ready to pick-up the silk after attaching it to the frame. Cut a piece of silk about 1" larger all around than your frame, I usually iron it to remove any major wrinkles, not absolutely necessary, but it does make it easier to get the silk to lay flat on your work surface. Smooth it out as best you can, paying particular attention that the weave is as straight as possible. Silk applied with the weave on the bias is much more likely to cause warps. Now we need to dope the frame to seal the wood that will come in contact with the silk. Two or three coats is sufficient 50-50 dope/thinner is OK, or it can be a little thicker. When you have enough dope on the frame so that it is not soaking-in and stays wet all around, place it doped side down on the silk. The silk will adhere to the frame, and you can now use those cross piece handles to pick it up, turn it over and smooth any significant wrinkles while the dope is still wet. Being sure that the silk is attached to the frame all the way around, we are now ready to pre-dope the silk.

I use 1 part dope, 2 parts thinner, it should drip from the brush, if it doesn't, add more thinner. I have never used anything other than a regular inexpensive camel hair brush, about 1", but I do use the side of the brush

in easy strokes. Don't worry about the dope running through, that's what it's supposed to do, thin dope will not make an unsightly blob on the underside. One coat is all we need for sizing, when dry, using a razorblade cut around the inside of the frame and remove. You now have a piece of silk that's ready to cover your model that behaves like no silk you have ever used. You can cut precise edges and shapes (like silkspan or tissue), something that's hard to do with unsized silk. If you haven't waited too long, it's easy to remove the silk still on the frame by just pulling it off. If it's really stuck, a little acetone or thinner will release it, now you can repeat the process in pre-doping the next panel. One frame should be sufficient to do all the wing panels for example.

Covering; Just lay the piece of silk over the area you wish to cover, spray lightly with water and lightly pull to remove any wrinkles, because you have pre-doped the structure and the silk all it takes to bond the two is a little thinner brushed around the edges where the silk touches the structure. Undercambered wings require heavy dope or thinned glue applied to the bottom of each rib and at dihedral breaks to adhere the silk. Acetone works best here because it's a better solvent and will form a faster bond. Compound curved surfaces, wing tips, Etc, can be easily covered by simply brushing thinner over the surface to release the sizing and let the silk conform as it normally does. Yes, the silk will dry taut after covering wet, one thin coat of dope does not seal the fibers to prevent the silk from shrinking normally. When you're ready to dope after covering, I prefer several thin coats to one thick one to fill the weave. Remember, if the dope starts to "run through" and make a blob on the inside, it's not because it's too thin, it's too thick!. As I said earlier, I use the side of the brush with light strokes until the weave is filled, then you can brush normally. I hope I haven't confused you too badly, at first it sounds like a lot of extra work, but in reality I believe it actually saves time. Applying the silk really goes fast and the results are totally predictable with a smooth covering job every time and with much less warping tendency. Try it , I think you will like it!

 

Elmer Nelson of Phoenix, AZ MAC wrote [regarding Starline dye]:

I've used the following method on polyspan and it works well. I would assume the same would apply to silk.

Mix one part dye with 10 parts nitrate thinner and add one part of nitrate dope. The color coat is sprayed on after a build up of dope.

 

Jim Kutkuhn, Casa Grande, AZ of Phoenix MAC told me:

Instead of using toilet paper, use a dryer anti-static sheet [we must assume this would be a used sheet]. The material won’t come apart like toilet paper, and if you lightly iron the sheet, it will lay flat and work quite well.

Jim and I also talked about using a spray gun to apply early clear coats. I'm wondering if thinning clear with a fast thinner like acetone would make sprayed clear dry almost instantly, on the surface!

A few weeks ago, I used acrylic artist’s ink to color the backside of clear Micafilm – because the colored material won’t be exposed to any kind of fuel, almost anything can be used. Some years ago, I used hardware store cream colored spraycan enamel on clear Micafilm to get a ‘doped linen’ effect for a 30s homebuilt.

The ink was sprayed using a Harbor Freight $16 touch-up spray gun. 2 coats were necessary to get a good color – which looks quite like red silkspan used to look. The color is on the back of the material and has no effect on the covering process because Balsarite, or SIG Stixit is used as the heat activated adhesive. BTW: the ink becomes waterproof when dry.

So, there we have the collective thoughts on covering with silk. My model won’t be covered for a few weeks, so I can’t tell you what I used yet. If you have any more ideas, let me know and we’ll add your thoughts to this list.