Covering Fuses with Silkspan

I want to start taking my finishes to the next level, "hiding the grain". I keep reading about people covering the fuses with silkspan and I would like some tips on how to do it? What weight of silkspan and the process for applying it??

Can someone provide some help??

I use "OO" or medium weight to cover with. It all depends on what I have on hand. You can use any of the types of dope to attach it. I am getting ready to try attaching it with polycrylic that several people have written about on here.
I, personnaly, use nitrate to attach all my silkspan. I put on enough coats to make the wood have a *sheen* to it. Sand with 400 or such to remove the fuzzies, and attach the silkspan wet. I try to cut the pieces as close to the correct size as I can, but you can sand the overlaps. When it is completely covered, start using what ever clear dope (or finish) to seal the silkspan, then use a filler. Zinc Sterate from Randy Smith (Aero Products) is an excellent material to mix in the dope for a filler. Talc and other similar items can also be used, but the ZS is the lightest filler I know of.
I use the nitrate since it is the easiest to *stick* silkspan with.
I also believe it *shrinks* less. Just do not use nitrate over butyrate dope.
Bill

read Godzilla's finishing post in the product reviews. Excellent overall job.

Silkspan, even 00, adds a lot of weight. Three coats of wb polyurethane are quite a bit lighter and usually can be sanded down to a grain-free finish. If not, one coat of wb polyurethane with 10-20% corn starch mixed in will fill any remaining marks and still be lighter. Silkspan adds next to no strength to the part, so it is just a thick, heavy filler.

Ferocious:

While I've nothing against more modern finishes, I have to disagree on a couple of points about dope and silkspan. In my experience, it does not add excess weight (unless 1/2 an ounce total or less bothers you) after properly applied and sanded out, and it DOES add significant strength.

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"You can't say that civilization don't advance . . . for in every war they kill you a new way." -Will Rogers

LAST EDITED ON Oct-30-03 AT 09:34 AM (CST)
 
>Ferocious:
>
>While I've nothing against more modern finishes, I have to
>disagree on a couple of points about dope and silkspan. In
>my experience, it does not add excess weight (unless 1/2 an
>ounce total or less bothers you) after properly applied and
>sanded out, and it DOES add significant strength.
>
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>
>"You can't say that civilization don't advance . . . for in
>every war they kill you a new way." -Will Rogers
Gotta agree.

If anyone has built an I-beam wing and twisted it before and after the silkspan has been applied they wouldn't even be tempted to say it adds no significant strength. I also agree that weight isn't a significant factor. In a good finish the grain has to be either filled (which really does add almost no strength but can be heavy if not conscientiously applied and sanded) or covered. The material necessary to fill the silkspan is much less than that required to fill a porous balsa skin.

Even better, in my opinion, is the use of the ultra light cf veil ( 0.2 oz per square yard) applied dry over a couple of coats of dope. Just brush on a thin mixture of dope through the stuff and it adheres to the dope beneath it. Doesn't do quite as good a job as silkspan at going around compound curves but it is so thin and sands so easily (until it has two or more coats of dope applied over it) than you can slit and overlap it in areas where it won't quite follow the curves.

Finally, for very curved areas like cowlings and wheel pants I strongly encourage the use of 3/4 oz glass cloth applied dry over a couple of coats of dope. Same application method as the veil. The beauty of the cloth is it can adapt to some pretty amazing shapes AND, when applied with dope vice resin or epoxy it can literally be dabbed and brushed into place with a soft paint brush without sticking and pulling like it does with the resins. I actually cloth the inside of cowlings with this stuff and it isn't even very hard to do.

>read Godzilla's finishing post in the product reviews.
>Excellent overall job.
>
>Silkspan, even 00, adds a lot of weight. Three coats of wb
>polyurethane are quite a bit lighter and usually can be
>sanded down to a grain-free finish. If not, one coat of wb
>polyurethane with 10-20% corn starch mixed in will fill any
>remaining marks and still be lighter. Silkspan adds next to
>no strength to the part, so it is just a thick, heavy
>filler.

I have to disagree with Phil on this one. Now I have never used the Polyurethane method so cannot comment about this. However, when Bob Hunt used the epoxy base without using silkspan he noticed that it eventually sank into the grain (or the grain raised) over time. My experiences using Bob's method using epoxy as a base (with no silkspan) followed with dope/talc filler were the same. It looked good at first. A year or so later the grain was very visible. I would be a bit concerned that the polurethane might do the same over time without some base material (either silkspan or carbon fiber veil) being used with it. I note that Brad intends to use the carbon fiber veil on his next ship with the polyurethane.

As to strength, the silkspan adds tremendous strength to the wood. In fact, the strength increase of silkspan, compared to the carbon fiber veil method, is just about equal.

And as to weight gain, starting with bare wood and then adding dope and covering with silkspan will add about one ounce total if done properly. One ounce! And the polyurethane has got to weigh something. In either case you add the same fillers (the filler coat may add another ounce) and final finishes over these.

I don't see where you are gaining a weight advantage or a strength advantage using the polyurethane without the silkspan, and I would be concerned about long term durability with the polyurethane unless you use a lot of buildup (weight).

Again, this is not a knock on the polyurethane method as I have not tried it or observed its long term durability, but I do know of the strength and long term durability of dope and silkspan and also that when done properly the weight increase is very minimal.

So using Zinc Sterate works better than the sanding sealer? Also, when I add Zinc Sterate to the dope how much do I add? Also, who's a good source for 00 silkspan?
How much of an overlap on the seems do I need when covering the fuse?

Sorry for the "twenty questions" but I really appreciate the help!!

>So using Zinc Sterate works better than the sanding sealer?

Sig sanding sealer appears to be filled with zinc stearate.

I usually mix a tablespoon of ZS to a pint of dope. Could probably use less. You can use the commercially available fillers and sanding sealers on your initial efforts, they will work. I know thast Bill Werwage used to "SPRAY" Aerogloss *filler* on his planes and they were gorgeous. I just have several quart cans of dope on hand so it is easy to mix a filler.
I get my silspan from an older hobby shop, but I think Brodak or others can supply "light weight" silkspan.
Bill

>I have to disagree with Phil on this one.

I have to agree with Len on this one. From my experiance, everything he said is right on. Even though PC is much better alone than epoxy because it treats the wood. I haven't seen much grain reappear on bare wood planes treated with PC, it is truly a unique product.

Please define "heavy".

Weight is not he only criteria for a finish, it is simply one of the criteria. Along with weight there is strength, flexibility, ease of use, surface hardness, longevity, color hold out, gloss holdout, and fuel proofness.

I have a little database of info on finishes from other guys planes that I keep in my head. The most rugged finishes are by far the auto finishes, although some of the prettiest (when they are new) are the dope finishes (mostly due to all of that hand rubbing). The longest lasting finishes are the carbon covered planes (auto and dope), followed by the PC covered planes. I think the PC and silkspan covered planes may be as good as the carbon covered planes, but only time will tell. My hope is that the PC and carbon covered planes will be the best yet.

What about the silkspan weight? I've read that light weight silkspan needs more dope to fill, so medium is better because it fills quicker and provides an overall lighter finish.
Your Thoughts?

LAST EDITED ON Oct-27-03 AT 12:56 PM (CST)
 
Hi George,
Can't say that I have weighted the finish on each type, so I guess someone else can give you that info.
But I have used both grades, and also "heavy" silkspan, and I truly believe that the greatest weight savings is in wood selection.
Bill

Where you are covering wood and not open space, I think you'll find that silkspan fills quite easily and quickly. I use medium or heavy grade for this, but I also cover the nose area first with .56 oz glass cloth (nitrate dope binder) and the silkspan and dope is over this.

It is not terribly heavy, you sand away much of what you apply in dope/filler.

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"Eat a live toad in the morning and nothing worse will happen to you for the rest of the day."

Could it be that the problem of the woodgrain reappearing is because the dope continues to shrink. The results might be different if he had used a paint like K&B epoxy. Using auto paint which don't continue to shrink afte application especially the two part paints might not have the problem of the grain showing up again.

I used plyspan to cover the fuselage of a profile painted with auto lacquer. The plyspan fills a lot quicker than silkspan, however there seems to be almost no strength added. The problems are the smaller size of the sheets, and not tearing the wet plyspan. I apply it wet just like silkspan and if you aren't extremely careful it'll disintegrate in you hands.

What weight is the silkspan that comes in the Sig kits? I have a ton of that stuff and thought I would try it.

Thanks for all of the great info!
John

I'm sure it is not the lightweight stuff, probably medium weight. It will work just fine. You'll see as soon as you try it, there's nothing much to it.

A nice thing about dope/silkspan is that it is not that critical or difficult to apply. It can be patched; over balsa sheet overlaps and even wrinkles will disappear into the finish as it is sanded and more dope/filler is applied. (By contrast, sand into polyspan or CF cloth and you have a mess that is tough to fix.)

It's a wonderful, low tech COMPOSITE technique.

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"Even if you're on the right track, you'll get run over if you just sit there." -Will Rogers

The Sig Plyspan I use is about the same as SGM, small gas model silkspan of the old days. It has plenty of wet strength and seems perhaps a little "springier' than the old silkspan. There is some confusion about which of the various Sig products we are talking about here. The Light Plyspan is like tissue. I'm not sure Sig is consistant in its nomenclature

So far as adding strength, do this experiment. Take a corehouse or similar foam wing, wetcover it with silkspan or Plyspan, using thinned down titebond. While it is drying take an uncovered wing and break it in two. After the silkspanned wing dries, break it in two. See what I mean?

I cover fuselages dry,just doping the pieces of silkspan down around the edges. Then I watershrink. This will give you a much smoother surface than just doping the silkspan down, either wet or dry. Also you can make buttjoints which will stay together doing it this way. Then stick the silkspan down with thinner in the undoped areas. Then proceed to dope and fill as you will.

I like Jap tissue for covering solid surfaces better than silkspan. Years ago (early '70s) I took a Starduster X (free flight) fuselage covered in colored tissue and clear dope and laid it across two lawn chairs and lifted myself off the ground with it using both arms and it neither bent nor broke. Now I'll grant you I only weighed about 125 pounds at the time, but this convinced me that tissue (or silk) and dope covered balsa is VERY strong...

Considering the Starduster X is a very tight box fuselage with internal braces, this is no surprise. Gee, fer a Texan ya'll sure are a light weight. Heh, heh. Ya gotta meet my son in Red Oak, a real big boy.
However, as far as covering the fuselage with silkspan, this has been advocated and used since the 50's at least, based on articles in Flying Models, MAN, Air Trails from that era. Simply put, it ain't broke and it woiks.

>Considering the Starduster X is a very tight box fuselage
>with internal braces, this is no surprise. Gee, fer a Texan
>ya'll sure are a light weight. Heh, heh. Ya gotta meet my
>son in Red Oak, a real big boy.
>However, as far as covering the fuselage with silkspan, this
>has been advocated and used since the 50's at least, based
>on articles in Flying Models, MAN, Air Trails from that era.
> Simply put, it ain't broke and it woiks.

That was in 1974-6...
You ain't seen me lately Chief- I look like a black bear on his last winter coat. I'm 5'6" and 240 pounds now- although I will grant it is spread out fairly evenly over the framework so I don't look like a complete dork- although there are those both here and elsewhere who will debate that...

See ya!'

I agree with you Jake! Though all posts are really all very wonderful and workable (don't disagree with experience) Speaking of which. A.A.M-June, 1960 "My Mustang was finished with two coats of Randolph non-tautening butyrate dope to seal the wood. I then covered the entire airplane, except the fiberglass areas, with jap tissue and two more coats of the Randolph dope......." W.A.Rabe Jr.
I am enjoying my finishing process after many years frustration--Learning process. Dave S.

I apply light silkspan dry to fuselages which have been clear coated with butyrate and sanded (up through #400) enough times to give a bit of a sheen. I apply thinner over the dry (unshrunk) silkspan, with more clear added where necessary, and never see a wrinkle. The thinner just seems to pin it down. Result is a smooth finish. The new "silkspan" does not adhere as well as what I purchased a couple years back (see that thread) - probably due to the smaller fibers, greater amount of adhesive matrix in its structure, and resultant smaller surface area (porosity). Therefore, early subsequent coats seem to need to evaporate quicker to ensure that paper does not lift/float. More thinned coats ( brushed on, 60/40) seem to work. I haven't seen a need to dampen the silkspan. I am also not convinced that filler helps in the long run, unless it's lighter than the dope. The only problem with my finishes has been my inability to spray well (yet), leaving a rougher surface than my shiny brushed clear coats, wet sanded with #400-600 and Prep-Sol. FWIW.

SK

I'm less sophisticated about it. I always wet silkspan, even soaking wet for application over clear dope, and pull out wrinkles. And after several clear dope coats, I apply a couple of very heavy filler/clear coats, which get sanded almost completely away. In the process, the "fuzz" and "oatmeal" surface on the silkspan disappears and it takes on a reflective, smooth sheen. Before I apply any blocking or color coats, the surface is already reflective and flat. And the filler coats are transparent, having been mostly sanded away. It's a magic transformation.

In spite of its smell, I find that the clear Prepsol is an excellent wet sanding agent; you can get good life out of a finer grade of sandpaper. Although some say they use Windex, I've found that water based solvents damage the silkspan surface, which actually gets sanded into very slightly. Prepsol also dries out the skin on the hands, although I ignore that problem as well.

As I said, silkspan application is not very critical and is forgiving and repairable. I like that.

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"Man always travels along precipices. His truest obligation is to keep his balance." -Jose Ortega Gasset

silkspan and dope is an overall lighter finish. Dried dope is 20-30% less dense than the same thickness film of PC, and epoxy is 20-30% denser yet. It is very hard to beat a dope finish for lightness and good appearance. It's less dense primarily because it is porous. That is why oil from the exhaust eventually soaks into the dope. When using polycrylic the silkspan automatically sets a minimum thickness for the paint film- it ends up saturated and essentially a solid film of PC. The only way to get it thinner is to sand, sand, sand, after it has dried. Just PC by itself turns out a much thinner, lighter film, so it weighs less.

After all the planes I've crashed, I can't say that silkspan did much if anything to make them stronger. They all broke, stabs, wings, rudders. But 3/4 oz. glass makes a huge difference in strength.

I also did some experimenting with stabs for stiffness. A sheet stab covered with silkspan(polycrylic finish) bends just about the same as one simply painted. Applying a strip of glass and epoxy along the trailing edge, top and bottom, approximately halves the amount it will bend under a given load. You can flex the stab and see the amount of bend literally double just where the glass stops.

LAST EDITED ON Oct-28-03 AT 10:51 AM (CST)
 
>I also did some experimenting with stabs for stiffness. A
>sheet stab covered with silkspan(polycrylic finish) bends
>just about the same as one simply painted. Applying a strip
>of glass and epoxy along the trailing edge, top and bottom,
>approximately halves the amount it will bend under a given
>load. You can flex the stab and see the amount of bend
>literally double just where the glass stops.

This is what Bill Wilson found. There was no strength advantage (in bending) to using silkspan with PC. He did exactly the same tests.

I think what Phil said is right on about the layer thickness of PC and silkspan. That is exactly why I use it in that manner, I want the build up. I just sand more at the PC stage, which is fine with me. I would rather sand PC and Zinc Stearate in my shop than primer in the garage...

The glass cloth findings are interesting. Also, could you please test the .2 oz carbon with PC?

LAST EDITED ON Oct-28-03 AT 10:54 AM (CST)
 
If you take a sheet of balsa and strip off some short pieces. Dope one (or two), then cover one or two with silkspan and dope and one or two with .2 ounce carbon fiber and dope. The covered pieces will be much stronger than the uncovered pieces. However, in a crash they are going to break. Now, epoxy and carbon fiber strands will get stronger, stiffer, and heavier. Cover the strips of balsa with glass or carbon fiber cloth and epoxy and they will be much stronger (and heavier). They will break, however, in a crash. But adding narrow pieces of carbon fiber to the structure will strengthen it and stiffen it. It can then be covered and finished your favorite way.

The key, I believe, is to add structure where it is needed to either prevent bending or breaking in the air. But we are never going to keep it from breaking in a crash unless we make them like a brick. And even bricks break when dropped.

Oh, I was going to add and forgot, If you do the above test, I would be interested to see how the polycrilic fares against the others. How does it compare in weight? How does it compare in strength before it breaks?

>The key, I believe, is to add structure where it is needed
>to either prevent bending or breaking in the air. But we
>are never going to keep it from breaking in a crash unless
>we make them like a brick. And even bricks break when
>dropped.

I agree, Len. A couple of former Walker Cup winners and World Champions made the point to me early on that we can't build a good stunt plane that will survive a crash. They should "explode" on impact!
Bill

>silkspan and dope is an overall lighter finish.

I doubt that it is if comparing mil to mil thickness. "Coats" cannot be used as a measure becasue the PC and epoxy are substantially higher solids (more mils per coat).

You might need three coats of nitrate dope to equal one coat of PC, but at the same mils I bet they are the same. Plastic is plastic.

>
>As to strength, the silkspan adds tremendous strength to the
>wood. In fact, the strength increase of silkspan, compared
>to the carbon fiber veil method, is just about equal.

I can't dispute this statement Len, but...

Ask Keith Trostle about the CF veil covered fuse on his first Chizler that met its demise straight into concrete following a control system failure. The very front was smushed around the engine, the wing and tail destroyed and detached from the fuse...but the rest of the fuselage was pretty much intact!

I've crashed a few silkspan covered fuses in similar fashion and had nothing much but puzzle parts after the fact.

Silkspan adds next to no strength to the part, so it is just a thick, heavy filler. (Ferocious)

Phil, I have drawn exactly the opposite conclusion. An entire sheet of 00 is only around 10 grams or so. And it does add strength to a full fuselage, kind of in a monocoque way. Agree that it may not be helpful, particularly, on a profile.

>Silkspan adds next to no strength to the part, so it is just
>a thick, heavy filler. (Ferocious)
>
>Phil, I have drawn exactly the opposite conclusion. An
>entire sheet of 00 is only around 10 grams or so. And it
>does add strength to a full fuselage, kind of in a monocoque
>way. Agree that it may not be helpful, particularly, on a
>profile.
>
>Currell

As a binder to the finish it does add strength. Take a 1/6 thich, 3 inch wide piece of balsa. Cut off a section and split it lengthwise into equal 1.5 inch widths. Then cover one piece top and bottom with silk span and dope (or whatever) and merely dope the other. Add equal amounts of finish so they weigh the same. Then try to break them using some sort of scale for reference. You will see that the silkspan covered side is significantly stronger. (Using carbon fiber matt or other materials will produce similar results, and carbon fiber strand would produce even greater strength in the direction of the strands.)

Any takers on the test?

Been there, done that. I tried this with the Polycrylic stuff when I first learned about it from Jim Pollock. STRONGGG. And it was very light.

>I keep reading about people ""covering the
>fuses with "*silkspan*" and I would like some tips on how to do
>it?"" What weight of silkspan and the process for applying
>it??
>
>Can someone provide some help??
>
>Thanks
>JOhn

Hi John,
As I guess you figured out, there are a quadzillion ways and ideas on how to finish our models. As for your *original request* I think the silkspan method works, and works well.
Experiment with your techniques, you will most likely develope your own favorite methods as do all of us.
Good luck!
Bill

John
If your interested in building a slick plane, get a digital gram scale
HUH?
Here,s the deal. A light ,straight ,ugly plane flies so much better
than a beautiful , heavy one.(Ask me how I know).
You CAN have a nice looking light plane , but it requires attention
to all areas of the plane.
I double cover my wings ( 2 layers) with silkspan.
I fiberglass at least the first half of my plane,s fuse .Al Rabe,s
mustang. was completly covered in glass.
How much weight does this stuff add? I ain,t gonna tell ya.
Why ? Cause it depends on how it is applied& by whom.
Get a good scale
Do a sample sheet, or better an old clean dead plane
weigh what youve done and you will know what is and what ain,t
working for YOU.
I tried this on my last plane and was amazed at the truth
about what was heavy and what was,nt: inc. glues.
Of course its a must for checking kit wood.
Tom Morris has a scale for under 40 bucks thats a jewel.
Hope this helps