Musings on Profile Fuselage "Silkspan"

This probably won't be news to many, but I thought I'd just comment on covering my profile fuselage. I had originally planned to use laminating resin and carbon veil, but got cold feet and reverted to butyrate and light silkspan - or whatever that stuff is that passes for it these days.

So I did my normal thing: alternately brushed on dope and sanded with 300, 400, and ultimately 600 (wet w/'Prepsol'), until the surface was smooth with minimal grain. That added only about 1/4 oz of weight. Plan is (was?) to adhere the "silkspan" by brushing in thinner, with the clear butyrate handy for any difficult parts. Usually I then finish as most others do.

I found that the best (strongest) light stuff I have is some Brodak material that came folded. It has a definite strongest (grain) direction, unlike my newest other stuff from SIG, K&S, etc. Experience has shown though that creases are difficult to get to lie flat in doped applications. So I tried ironing the stuff. Now it's crinkly and ripply - not so good...so much for that.

So I went back to the other (rolled) weaker stuff. Here I found that strength didn't seem adequate to justify the weight and complication, especially since the matrix is so prominent. I remember this stuff's tendency to lift (float?) during application.

I checked went further back and found what I think to be the best medium grade I have on hand (.03" - .035" vs. about .02+" for the Brodak material). It may even be a left-over sheet of older material; it has grain direction, a smoother side, and fewer blotches. However I found that the grain runs in the shorter (23.5") direction. The strength in this direction seems to be much greater than lengthwise, using the venerable tear test without a scale. I don't even want to dig out the rest of the junk to test it now, remembering how much I dislike it, and I have forgotten it's grain direction. While two pieces would be OK for these fuselage sides, it does seem that we modelers are at the bottom of the food chain when it comes to this paper product, and papering large span models is not likely to be in the cards for at least some of these rolls. Finally, I really doubt the usefullness of this stuff for added stiffness vs. weight.

So if I'm using medium thickness material, maybe I'll just go back and use the carbon veil with dope. I'm not looking forward to what happens to its cohesion when I hit it with thinner or thinned butyrate. 'shoulda used laminating resin and just followed my original plan!

...or maybe I'll just complete the dope finish without a laminate. It's pretty stiff as is.

'just thinking some "out loud". When I was a kid, I didn't have to think about these things. I didn't even know how to finish well...AND THE SILKSPAN WAS REAL. Ha!

'really just grumbling a bit this A.M. It'll all work well enough anyway, and after it's over, I'll forget what all's hidden underneath - except the weight...

Did I mention that I hate the new "silkspan"? Yeah, you knew that.

SK

When covering fueselage with silkspan, stick the pieces down around the edges. Let dry, water shrink, let dry, stick down with thinner. Gives a really smooth surface. Try Sig plyspan. It is pretty close to the old time silkspan. Personally, having seen some "silkspan" I would not buy silkspan without looking at it first.

Being an engineer (a scientist with thumbs) I find it very practical to simply take the bull by the horns. I don't worry about imperfections or folds in my silkspan. I use a medium grade, cut to approximate size, and lay it over the fuselage side. Then I spray it with water until it is completely soaked (not just damp), and stretch it to remove any wrinkles.

While the silkspan is still very wet, I begin brushing on heavy coats of clear dope. As it begins to adhere (still wet with water), I rub the area with my hand (ungloved!) which further smooths the silkspan, and eliminates any little puckers or wrinkles.

On edges I make sure there is a good adhesion and rub them down well. The excess silkspan is easily removed when the dope dries with a gentle swipe of the sandpaper. After the silkspan is properly shaped and adhered, and things are dry again, I apply one or two more coats of clear dope. The filler coats are next, in my case nitrate with ordinary talc.

After all is said and done, and the sanded out finish is satisfactory, ready for color, with very little weight added, will never pop and show grain, and (surprisingly perhaps) has some useful amount of stiffness and rigidity added.

(The Cowboy Way again, as taught to me by Bill Melton..)

[photo not recovered: 440deeec35ce0bef.jpg]

"Before you put on a frown, make absolutely sure there are no smiles available.
-- Jim Beggs

Serge,
This may be a redundant question, but have you considered using .5 oz fiberglass cloth applied with dope. Yes it's a little heavier than silkspan or carbon veil but the strength and stiffness even applied with dope is phenominally greater, and it finishes very nicely. I've done several profile fuselages with it and the stiffness added to the rear of the fuselage is well worth the effort.
I typically use nitrate dope because it's adhesion qualities are significantly higher and less flexible than butyrate but it may be too late for that in your case if you've already applied butyrate.
I have used butyrate and it works well. I apply the glass cloth using a method I saw in a Magazine some time ago by spraying a very light coat of 3M77 (sprayed into the air above the surface). it gives just enough adhesion to hold the cloth in place and smooth it out before applying the dope. I've also used this method to apply the glass cloth with CYA. This works very well and is stiff, stiff, stiff (the fumes of course are atrocious). However it is a little heavier and somewhat more difficult to finish.
Like I said this may be a redundant suggestion!

Thanks, Randy and Jim!

I did buy some plyspan for just that reason: it looked to be made from the old style silkspan, having been on the shelf of a now closed LHS for some years. I covered the wing for this model with it. Then I learned that it fades and apparently tears with age (from Ted). Still, if I had any left, I'd consider it for the fuselage.

Randy's fiberglass idea may be just what I should do though. Yes, I have some, and while rethinking things this morning, I had forgotten it. It would not come apart in the solvent, and it would conform to the compound curves better than carbon veil.

I'm still thinking...and thanks for the hints on technique.

SK

I have been well satisfied with Sig plyspan so far as strength and durability go. I have used mostly the yellow, and yes, it does fade. So far a covering a fuselage goes, silk is wonderful to use. Silk is easier to use than fiberglass, but probably not as strong.

>I found that the best (strongest) light stuff I have is some
>###### material ... it's crinkly and ripply - not so good...
>SK
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Try covering your wing with that awful stuff and you'll find the same result. Been there/done that. Replaced it with some nice old K-S silkspan sent by good friends.

How about Polycrylic instead of dope?

Serge----I feel your pain. nothing is like the old K&S silkspan. I hate all of it, nothing is even close.

How about silk?

I've been using medium and light Brodak stuff and Sig. I put three or four coats of dope on the fuse, sanding lightly between coats. Then I apply the silkspan damp. The wet silkspan seems to adhere to the surface readily, kind of a static cling, working out most of the wrinkles. The wet silkspan is smoothed out flat. I then dope on top while the surface is still wet smoothing out the rest of the wrinkles using my fingers. Don't usually have a problem. Then I work on the primer coats using Brodak primer, sanding out low spots and imperfections. Gray coat on top of that covers irregularities in color. Color coat on top of that etc. Last few profiles came out looking good. Carbon veil or fiberglass is no doubt stronger and more rigid. So far didn't notice much tail flex on my Buster, Galaxy or Twisters. The Buster in any case gets flown fast. The Twisters are rigid enough long sticks.

I know there's been lots of talk of the inferiority of modern silkspan. Well, honestly, I haven't been able to figure out a grain pattern on the new stuff or old stuff. The new stuff does look kind of blotchy. When applied the way I have been doing it, can't see any blotches or unevenness of color. In fact my last profile sitting on the work bench awaiting this and that has a fairly soft wood fuse. It got knocked around some, requiring plastic wood work to even the surface out. That left areas of yellow stain. After the doping, silkspan, primer and gray coat, the finish is smooth and even. Color coat should be no problem.

Well, there you go, a duffers eye view.

Best,

LAST EDITED ON May-17-06 AT 06:38 AM (CST)
 
Progress is our most important problem

I dabbled with that on 2 or 3 planes.

Covering the black color seems to require a lot more paint than I like to use. One answer might be to just build models with black fuselages. The last one was red and it took too much to cover the carbon.

Sort of off Topic (maybe). I really like the Brodak medium silkspan but unfortunately they have discontinued the item. So now I am searching for a suitable material myself.

Believe it or not red is a very transparent color. The best way to use red is to use a lighter color under it to "opaque" the substructure. Full scale used the aluminum primer on fabric airplanes to provide some UV protection and also to "opaque" the covering for the color coats.

Thanks for your thoughts, Dennis.

>The new stuff does look kind of blotchy. When applied the way I have been doing it, can't
>see any blotches or unevenness of color.

The reason I mentioned the blotches was mostly in reference to observations in silkspan posts a while back. The blotches seem to indicate more and unevenly distributed matrix holding the "silkspan" strands together. I think that this is a reason that the newer material seems to want to float, rather than adhere as well to the surface, as subsequent coats of dope are applied. There probably is less surface area (less exposure of strands) down there too. Anyway, this may also account for the harder/stiffer, crinkly nature of this material. Although it ultimately gets quite tight on open bays due to dope shrinkage, it wrinkles up much more when dope is reapplied, even though no wrinkles were present before dope was ever applied to the bays. Ah, but don't get me started! Ha!

Well, anyway, we had good silkspan for more than half a century. 2002 seemed to be the transition year for my sources. I wonder whether anyone anywhere still makes the good stuff...

SK

Serge,

Did I read your first post correctly, you go to a 600 grit surface before applying silkspan?

I usually only do back-row (sport) finishes, but I only sand to 220 before applying silkspan. I wonder what others do?

For my purposes, I apply the silkspan wet and use thinner or very thin dope to adhere the 'span from the bottom. After it dries and "shrinks", I use a couple more coats of dope, then filler and sanding as needed until ready for color. I only go up to 300 grit before color.

Of course there are other ways to do the same task.

How do you front row (or near) finishers prep?

>Did I read your first post correctly, you go to a 600 grit surface before applying silkspan?

Sometimes. It's not that I believe it necessary, but just that when most of the balsa is sealed, I can use the Prep-Sol for wet sanding and sand fast without clogging the paper. The coarser grades seem to sand awfully fast under these conditions, and I don't want to dig into bare balsa just before applying silkspan or whatever. It's easy to see how much grain is left then too. I was worried that this might result in too much base dope, but the total gain this time was less than 1/4 oz - so, what the heck, I just did it. Now, I'll just use thinner through the laminate, with the thinned dope on stand-by for any edge difficulties during the initial application - today??. It's a plan anyway.

SK

Well, today, between those other things that make up "life", I finally bit the bullet. After normal procrastination, I applied .56 oz/yd^2 fiberglass cloth with thinner over my smoothly doped fuselage surface.

The good news:

Wow, was that ever easy! The brush just adhered the cloth and moved the wrinkles right out of the way. The cloth conformed to some pretty drastic compound curves. I only had a problem of lifting in one small place and remedied it - I hope permanently - by reapplying thinner and a small cut. Flash sanded right off. The job went quickly, with no mess at all.

The not-so-promising news:

I had calculated a weight of only about .11-.12 oz total for the glass and had hoped for modest gain in butyrate dope weight. However, with considerably less that a full coat of clear (mostly edges to aid in sanding flash cleanly), the weight gain has been close to 1/4 oz already, and the cloth seems to need more to fill the weave than I expected. Perhaps I have found a way to create matter - that's it! Maybe that explains all the tunnage I've directed at the sky. I know some of that weight will gas off, but...ho-hum, what ELSE is new?

SK

LAST EDITED ON May-19-06 AT 07:59 AM (CST)
 
Hi Serge, long time. I would suggest a coat or 2 of full strength clear dope to insure adhesion (more then that and you'll start to get corners/fillets lifting anyway) then I'd use Duplicolor Scratch/Dent filling primer to fill the weave. Its light and builds fast. I bet you could have it done it 2 coats.