From: Brett Buck <buckbw@pacbell.net>
Subject: Re: Straight Dope?
Date: 28 Dec 1999
Newsgroups: rec.models.rockets

"Scott D. Orr" wrote:
>
> I've read a few different places that using airplane dope mixed 50/50
> with dope thinner makes a good sealer for fins and glider wings.
> Stine also recommends adding a few drops of castor oil to prevent
> warping resulting from shrinkage.
>
> My questions:  I've discovered that SIG makes a dope called
> "Lite-Coat" that, while a butyrate dope, is supposed to be lighter and
> doesn't shrink much (and it's gloss, to boot).  If I use this rather
> than regular dope, do I still need a 50/50 dope/thinner mix?  Do I
> need any thinner at all?  Second, do you think I can dispense with the
> castor oil?


   Lite-Coat works well without any plasticizer (like cator oil). It
still should probably be thinned before brushing or spraying
effectively. It is far, far less shinky than Supercoat, and will not
warp thin surfaces as easily.

      The best way to use this is to use it to attach Jap tissue or .2
oz graphite mat, and then fill over this. It takes far less filler than
trying to fill all the grain directly, and it doesn't have as much
tendency to show the grain later. Put down a couple of coats over the
bare wood, then use 75%/25% tinner/dope to hold the tissue down. Then
put on a few coats of 50/50 clear to seal it, then put the fillercoat
over the top. Sand it down, then let it sit for a few days, and then
prime/paint over the top.

    Lite-Coat is widely available, but a much superior product is
available from Brodak ( http://www.brodak.com/ ). This dope is
specifically formulates for C/L model airplanes by full-scale supplier
Randolph's. One of the problems with all the SIG stuff is that it never
completely cures, and stays slightly gummy for months. Brodak's does not
have this problem - you can put it on, sand 2 hours later, and get dust.
It's completely incompatible with SIG or Aerogloss.   I *very highly*
recommend it - it's not even close, its tremendously better!

    Brett

From: Brett Buck <brett.buck@lmco.com>
Subject: Re: Tissue Paper Sealing of Fins
Date: 06 May 1998
Newsgroups: rec.models.rockets

The Silent Observer wrote:
> If the tissue is to be sealed, that's
> generally done with nitrate or butyrate dope -- nitrate shrinks more,
> tautening the covering, but butyrate is chemically preferred for use
> with nitromethane-based fuels as in model airplanes.

    Actually, Sig Nitrate has much lower shrink than any other type of dope.
In order of degree of shrinkage (highest to lowest, number is approximate
relative shrinkage):

  Sig Supercoat Clear (butyrate)     10
  Aero-Gloss   (composition unknown, but known to have compatibility
properties of nitrate)                 6
  Sig Supercoat colors (butyrate, Litecoat with pigment)      5
  Sig Litecoat clear (butyrate)        5
  Sig Nitrate                         2
  Randolf's Nitrate                   2

    Over solid surfaced rocket fins, not much if any shrink is necessary,
although it usually doesn't hurt anything because the fins are so stiff. Over
open structures like built-up glider wings, some shrink is beneficial -
Lite-Coat works pretty well. SuperCoat has such high shrinkage that it should
be used sparingly only on very silod underlying structures. Another
unfortunate property is that is continues shrinking seemingly forever, and
even the most stout C/L stunt planes and R/C aircraft eventually succumb to
warps.

   Brett

From: Brett Buck <brett.buck@lmco.com>
Subject: Re: Tissue Paper Sealing of Fins
Date: 06 May 1998
Newsgroups: rec.models.rockets

The Silent Observer wrote:

> Interesting -- this is exactly the reverse of the "common knowledge"
> from when I was building and flying a lot of free flight models.  Then,
> the warning was against nitrate dopes without plasticizers, because they
> were the ones that would shrink forever, and butyrate was preferred for
> sealing light structures (after using nitrate and/or thinned cement to
> attach the tissue).

    Sig Supercoat clear is the biggest offender - I've seen 1/8" balsa
fusalage sides on C/L stunt planes look like starved horses (show the internal
formers) inside of 6 months, and it just keeps getting worse. It also has
relatively poor stick, so covering over concave sufaces like wing/fuselage
fillets results in it always pulling away and looking like its' got "the pox".
The best traditional dope finisher I know (a C/L stunt guy named Windy
Urtnowski) uses Litecoat/plasticizer all over, and then covers only the open
bays with spot application of SuperCoat, and then only one coat. Clear coat is
all Lite-Coat.
    LiteCoat and Supercoat colors have light shrink and can be used over FF
type structures without too much problem. The problem I had with Sig nitrate
on small FF was that it never shrunk enough to stiffen the structure, but it
did make the covering brittle.


> I do KNOW I vowed never to use AeroGloss again
> after trying to attach paper with it -- it wouldn't even stick the paper
> down, much less tighten it.

   Most people have made that same vow, for a variety of reasons. I don't
think it's made any more, but it's still widely available.
     I never had a problem sticking down the tissue with it (although nitrate
is much better), but it doesn't shrink much. In applying silkspan wet to weak
structures  the low-shrink really helps. If you want to to shrink it up, then
(depnding on which batch you get) you might be out of luck.
    The biggest problem with Aero-Gloss (and the others to a lesser extent) is
that no two batches are alike - sometimes they are not even compatible.
Aero-gloss has many horror stories associated with it - spend 6 months
filling, masking, inking, lettering, masking and painting colors on a C/L
stunt plane, get to the last coat of clear, run out, open a new can, spray  -
and get a million hairline cracks when it crazes the dope underneath.

    The general consensus is that the manufacturers have to continually change
solvents and plasticizers to meet the ever-changing whims of the EPA - so the
batch made last month won't work with the batch made this month.   Sig seems
to be much less affected than Aero-Gloss - but just recently it was noted that
new SIG Lite-Coat no longer seems to work over Sig nitrate (after years of no
problems).

   For most stuff, I try to avoid dope in general anymore when it can be
avoided - nitrate to stick down the covering material (almost always .2 oz/
sq. yd graphite mat, occasionally silkspan over open bays or when weight
doesn't matter), then switch to 2-part epoxy or automotive base/clear(like PPG
DBU series). For rocket fins, silkspan or graphite/nitrate, thinned model
magic filler, then automotive primer and testors solvent-based enamel. I have
had very mediocre results with the Testors water-based paints.

>
> Nitrate has the additional hazard of flammability even after it's fully
> dry, but that's mostly a problem on open frames, where there's nothing
> to absorb applied heat, and when attempting to use a flame to ignite a
> dethermalizer fuse.  >Don't< try this at home, kiddies -- this burned
> both skins off a 30-inch wing in about five seconds.  B(


     This is a serious concern, particularly with rockets. There's a reason
old nitrate film stock is so dangerous, and nitrate dope is just old nitrate
film stock in liquid form. I've never had a problem, but during the
application and even after it dries, it *could be*. Butyrate is somewhat
better, but only by comparison. For certain, don't apply in a room with a
water heater, pilot light, or other open ignition source. And use good
ventilation - both to avoid fires and avoid dain brammage.

     Brett

From: Brett Buck <brett.buck@lmco.com>
Subject: Re: Tissue Paper Sealing of Fins
Date: 06 May 1998
Newsgroups: rec.models.rockets

Bob Kaplow wrote:

> >     Stick with one brand and don't put nitrate over butyrate.
>
> Isn't it the other way around; don't put butyrate over nitrate?

    I'm pretty sure on this one; you can put nearly anything over nitrate but
only butyrate over butyrate.

> But to be
> safe, don't mix period.

    Even better idea - particularly with all the horror stories about
batch-to-batch incompatibility.

> No one has discussed tissue. For contest models, by all means use real "Jap"
> tissue, available from SIG or PECK. For sport models where weight isn't so
> critical, you can save some $$$ by using domestic tissue.

    Certainly true - make sure it's shiny-side out to minimize the amount of
dope needed to fill.

    I still think that .2 oz/sq yd graphite mat/notrate  is the way to go over
solid surfaces.

    Brett