Conference vmszoo::rc

1146.0. "Covering with Silk" by K::FISHER (Stop and Smell the Balsa!) Wed Nov 08 1989 09:10

From: [email protected] (Henry Pasternack)
Subject: Covering with silk.
Date: 7 Nov 89 00:38:14 GMT
Sender: [email protected]
 
 
   I have never had trouble shrinking silk when covering open-frame
models.  I wonder if your friend is applying the silk wet?  Also, for
best shrinkage, butyrate dope should be used.  Here's my technique,
which is pretty laborious, but gives great results:
 
   Prepare the model as though you intended to use Monokote.  That
means doing a quality sanding job and filling in any dings with putty.
Now brush on two coats of Aerogloss clear, sanding between coats.  The
first coat of clear will raise the grain and make the plane look
awful.  You want to sand down the fuzz and apply another coat.  After
the second coat, the fuzzing tendency should be under control and the
grain should be partially sealed.  Also, you will have established a
nice adhesive base for the covering.
 
   Now we start by covering the plane with tissue or light grade
silkspan.  I know it is an incredible nuisance to cover the plane
twice, but the results are worth it, as I shall explain later.  The
silkspan should go on wet; you'll need to cut it to size and then
spray the sheet with a plant sprayer.  It becomes very sticky and
fragile in this state, so you'll need some patience and care laying it
over the frame and pulling out the wrinkles.  If it starts to dry out,
give the paper another light misting.
 
   Once the silkspan is stretched smoothly, but not extremely
tightly, over the framework, you may begin to dope it down.  Brush
full-strength dope through the silkspan around the edges of the
framework.  The solvent will soften the underlying coating of dope and
make for good adhesion.  Use your fingers to rub down the paper and
make sure it sticks.  Brush some extra dope beyond the edges of the
wood to make it easy to trim later.
 
   After the dope and water have dried, the paper should have shrunk
up very nicely with absolutely no wrinkles.  Use a blade to trim the
excess paper away, or, alternatively, feather it off with some light
sandpaper.  Now go back and put some more dope around the edges,
rubbing them down to adhere the cut edges.  To this point, we have
only covered one side of the structure.  Don't cover the other side
until I say so.
 
   We now want to apply some clear dope to the paper in order to
strengthen it, waterproof it, and shrink it some more.  You should use
a brush, with 50% thinned Aerogloss.  Go carefully, so as not to
produce puddles on the inner side of the silkspan, and cover the
entire panel surface.  Make sure the paper is fully adhered to all wood
surfaces.  After the dope has dried, apply another coat or two in the
same manner.  Finish off with a very light sanding, taking care not to
cut through to the ribs.  Butyrate dope shrinks when it dries.  Later
on, successive coats will dramatically tighten the covering.
 
   You may now cover the underside of the structure in the same manner.
When you are done, you should have a very nicely covered framework with no
wrinkles and a pretty translucent-white color.  If you want, use a little
more dope and gentle sanding to cover up the seams.
 
   Why did we go to the trouble to covering with paper?  The answer is
that the paper provides a superb backing to the silk when it comes
time to fill the fabric.  You'll be able to seal the silk much more
easily because the additional coats of clear will flow against the
silkspan instead of beading up and plopping onto the bottom of the
opposite surface.  Anyway, we now move on to the silk part of the job.
 
   Silk should also be applied wet.  I do not like to use colored
silk, but it doesn't really matter.  The key to success is to get the
silk tight from the start.  Let's say you're covering a wing.  Cut a
piece of silk to size and spray it with water.  It instantly turns to
a tangled, unmanageable rag!  Get it straightened out, and lay it down
along the wing centerline.  Use the full-strength dope and brush it on
along the edge of the fabric, rubbing it in with your fingers.  Now
get some pins and stick them into the silk to hold it in place.
 
   You are about to develop some classic modeling skills.  You need to
work from the root to the tip, stretching the silk, pulling out the
wrinkles, and doping it down.  The dope has a definite working time:
let go too soon, and the silk slips; wait too long, and it sticks when
you don't want it to.  Be patient!  If the silk starts to dry, add
more water.  As you get it pulled out, stick pins in the underside of
the leading and trailing edges to hold it in place.  You want it to be
tight when wet!
 
   Eventually, the silk will be nicely stretched out and doped around
the edges.  You may wish you had three hands.  Your fingers will be
covered with glue and water from rubbing the fabric down.  You may wish
to remind yourself that you are participating in a time-honored tradition
of modeling that dates to the earliest days of aviation.
 
   You may be dismayed to discover that the silkspan has sagged
noticeably away from the silk due to moisture.  Don't panic; when
everything dries, the paper will pull back up in contact with the
silk.  Just remember to keep the dope off of the open framework during
the initial application stage.  Also, make sure the silk is applied
with the grain running spanwise, for maximum tightness.
 
   After everything is dry, trim the edges and apply another coat of
dope around the edges to seal the cloth in place.  Resist the urge at
this and all other steps to use a hairdryer to speed things along.  I
have had had troubles trying to quicken the drying process in this
way.
 
   Now we want to apply a coat or two of thinned dope to the silk.
Again, the silkspan will shrink away, but if you are sparing and
careful, it will come back up and bond to the silk.  Now you are home
free.  Finish off the remaining three panels of the wing.
 
   At this point, you should have a wing fully covered in silkspan and
silk.  You should have applied a coat or two of thinned clear to the
surface, and the cloth should already be impervious to air.  Now is
where the wonderful difference between Monokote and silk will begin to
make itself apparent.  You want to continue coating the wing carefully
with thinned dope.  Brush it on; don't use a can.  Sand lightly
between coats, more aggressively at the tips and edges.  After five
coats or so, all of the seams should have disappeared.  The wing
should be getting tighter and tighter with each coat, and it is now
completely fuelproofed.  Put the wing away for a couple of days to let
it thoroughly tighten up and to get the solvents out.
 
   Now it is time to paint.  I prefer to use Aerogloss at this point,
but enamel will work as well.  Unless you are an expert painter, I
recommend spray dope for best results.  The beauty of dope is that it
goes on in many thin, fast-drying coats.  Don't worry about weight;
this stuff is light compared to Formula-U.  Start with a white base
coat -- primer is not recommended.  You'll need four or more coats to
get good coverage.  Wet sand with 600 grit between coats, but please
go easy over the ribs.  After a while, a magical transformation will
occur, and the perfection of your silked wing will reveal itself.
Finish off with final colors and trim.  Dust yourself off, go breathe
some fresh air (you'll need it by now), and congratulate yourself on a
job well done.
 
   All of this is a tremendous amount or work, make no mistake.  When I tell
the hobby shop people I am going to silk and dope a plane, they look at me
and say, "Why?  Monokote looks so much better and is lighter, too."  Phooie!
The fabric covering is tighter than Monokote.  It doesn't show fingerprints.
It doesn't sag in the sun.  It never peels off.  It is very strong.  And it
weighs the same as Monokote (I weighed some from a busted plane).  Best of
all, it looks great.
 
   Trust me.
 
-Henry

    Back in the days when silk/silkspan and dope were the ONLY ways
    available to cover wings and other open structure, I pretty much
    followed Mr. Pasternack's methods when applying silk or Silron (a
    silk-rayon blend that may no longer be available...I believe it came
    from Royal Prod.'s).
    
    However, I _NEVER_ found it necessary to go to the incredible trouble
    of double-covering with a base of silkspan under the silk.  As Mr.
    Pasternack stated, the primary (to me, the _only_) reason for doing
    this is to aviod the problem of the dope seeping through the silk and
    puddling on the underside during the first coupla' coats.  I don't deny
    that this _IS_ a very real problem, one that can ruin an otherwise
    perfect covering job, but we came up with a (in my opinion) better
    solution thaN the time consuming double-covering method.
    
    I prepped the surfaces exactly as described, skipped the silkspan and
    went directly to the silk/silron.  Another caution here is that the
    surface(s) to be covered _must_ be sanded smooth and free of any
    sharpness/roughness as the wet silk will snag on these protuberances
    and can run just like a pair of pantyhose.  I always laid the silk over
    the structure dry, then dampened it with a spray bottle which was kept
    close at hand to re-wet the fabric as required 'til the job was done.
    Once the technique was learned, and it didn't take all that long...one
    or two wing panels and you were a pro...you could cover a model quite
    rapidly.
    
    Anyhoo, I never doped _ANY_ of the surfaces until _ALL_ surfaces had
    been covered...this to prevent warpage caused by the uneven shrinkage
    of the silk.  Once all covering was completed, here's what I did to
    stop the puddling os the first coupla' coats of dope.  Actually, I used
    two different techniques:
      1. I thinned the dope to the consistency of water, then brushed it on
    lightly using _LOTS_ of brush motion to spread the mixture out well.  I
    know, this sounds backwards as the super-thinned dope would much more
    readily soak through the fabric.  But here's the how-cum' of it:  Since
    it's virtually impossible to prevent the seepage and the thicker dope
    dries on the inside of the covering forming an unsightly bump, we said,
    OK, LET it soak through _BUT_ in such a thin mixture that it won't
    build up and cause a lump from beneath the surface.  Several _thin_
    coats were applied then the mixture was gradually thickened as the
    fabric's weave became filled/sealed.  From there, you were home free.
      2. This method worked just as well and saved a lot of time:  Once all
    covering was completed, we applied a spray starch, like Niagara, from a
    spray can, applied in light coats 'til the weave was filled.  Then
    carefully, the first coat(s) of moderately thinned dope were brushed
    on.  The starch barrier prevented seeping through and, after only a
    coat or two, you were on yer' way.
    
    The number one method was preferred if you intended to leave the silk
    clear/unpainted as the starch produced a somewhat milky appearance but
    this was of no consequence if you intended to paint with color dope.
    
    The primary disadvantage to the silk-and-dope method was the extreme
    shrinkage.  In places where you'd sanded a bit too thin, the sheeting
    would swell into grotesque eruptions or shrink down between
    formers/ribs giving the appearance of a starved horse.  Also, dope
    tended to erupt into unsightly bubbles/blisters in filet areas like
    around the stab/wing/fin-to-fuse junctions, just ruining an otherwise
    great appearing model.
    
    I might still consider using clear doped, colored silk on something
    like an oltimer or a sailplane but, with the great iron-on fabrics we
    have nowadays, e.g. super-shrink Coverite, any of the many -Tex's, even
    Sig Koverall, which require only a coat or two of K&B primer to be
    ready for paint, my question is...why bother?? 

      |                 
      | |      00	 Adios,      Al
    |_|_|      ( >o
      |    Z__(O_\_	(The Desert Rat)