Repairing the Crossfire Extreme - 201209

When I left off last time, I was
in the middle of the repairing/
refi nishing process of my
Crossfi re Extreme. In that column, I
indicated that refi nishing an existing
model is really not much less work than
simply building a new one. I’d like to
verify that notion here and now!
All kidding aside, the project has
progressed nicely and at this point I am
in the process of applying the color trim
to the model.
The original fi nish had been stripped
from the model down to the 0.2-ounce
carbon-fi ber mat that I had applied.
After the reconstruction and repairs
were complete, I decided to go ahead
and carefully block-sand the carbon
mat from the Crossfi re Extreme except
for the surface of the stabilizer and the
fi n, which looked perfect and ready to
receive a new fi nish. This process took
nearly 18 painstaking hours, but the
results were worth the effort.
I used an 11-inch-long Great Planes
Easy-Touch Bar Sander fi tted with 220-
grit sandpaper to remove the carbon
from the surface of the wings. This step
evened out the surface, because initially I
had sanded through the carbon in some
areas but not in others, leaving several
subtle bumps.
Block-sanding removed all of the
carbon and a bit of the balsa sheeting
that was on the wing. I had used 1/16-
inch sheeting on the wing and there was
plenty of material with which to work.
I used smaller blocks to remove the
carbon from the fuselage and stabilizer
areas. The result of the detailed sanding
was a stripped surface that was back
to virgin balsa. Not only would the
refi nishing be easier, but it removed
much weight.
The next step was to recover the
model with 0.2-ounce carbon mat.
Using this material on Stunt models
is not universally accepted. Some feel
The Cross re Extreme is
in a custom-made foam
 xture that allows easy
access to the hinge lines
for sanding and hinge
replacement. Bob had
yet to sand all of the 0.2-
ounce carbon mat from
the airframe at this point,
leaving a bumpy surface.
Hunt photo.
that it adds no more strength than does
covering the model with light grade
silkspan.
In my experience, not only does the
carbon add a tremendous amount of
strength to the airframe (especially
torsional rigidity to the fuselage), it is
also much easier to apply than silkspan
and can be overlapped and sanded
without leaving a bump on the model’s
surface at the overlap point.
Overlaps in silkspan leave noticeably
raised areas that are virtually impossible
to sand and can show through the fi nal
fi nish. Butt-joining silkspan to eliminate
this problem is a tedious process.
The carbon mat can be overlapped as
necessary, and the overlaps can be easily
sanded smooth when the dope has dried.
It works much like fi berglass in this
respect and the overlaps just disappear.
Another attribute of the carbon is that
when properly applied and sanded, it
acts like a fi ller coat. The carbon actually
fi lls and conceals the grain.
I prefer to use
a non-tautening
nitrate dope
to attach the
carbon fi ber to
fully sheeted
surfaces. I
believe that
it adheres to
and grips the
surface of the
wood better.
There is no
need on a foam
wing-equipped
model to use a
tautening-type
dope.
Had I built
the model
using an open
bay wing
that required
covering, then a tautening dope product
would have been necessary to achieve
proper shrinkage and surface tension of
the covering material. It makes sense to
me to not use any materials that shrink if I
don’t have to.
High-shrink dope will continue to dry
and “pull” over time, usually resulting in a
twisted airframe. Additionally, high-shrink
dope will continue to sink into the wood
over time and reveal grain.
To apply the carbon mat, I fi rst brush
on two coats of moderately thinned nontautening
clear dope. I buy Randolph
brand dope products from Aircraft Spruce
& Specialty in gallon quantities. You
would be surprised at how inexpensive
this dope is when ordered in quantity. Be
aware that nitrate dope has to be used
with nitrate thinner.
After the basecoats of clear nontautening
nitrate have dried, I lay a
piece of carbon mat onto the surface to
be covered and use 90% thinned dope
to attach it. The thinned dope easily
saturates the carbon and softens the
initial dope basecoats, ensuring a strong
carbon/wood bond.
If you use improperly thinned dope
for this process, the carbon will not
adhere properly and you will have areas
where it will delaminate. When these
areas are sanded later on, the paper
will cut through the carbon and leave
voids right down to the balsa surface.
These voids are diffi cult to fi nish and
will compromise the strength and grainfi
lling properties of the carbon, which is
the main reason we are using it.
After the model is fully covered with
the 0.2-ounce carbon mat, I apply a coat
of moderately thinned non-tautening
nitrate dope. When this coat has dried,
I lightly sand the surface smooth.
This is the point at which I sand the
overlapped areas fl at and smooth. If
you sand too aggressively, you will go
through the carbon, so be careful!
After this sanding, I apply two more
coats of non-tautening nitrate dope and
then carefully sand again. At this point
the surface of the carbon should be fi lled
and have a noticeable surface sheen. I
usually apply one or two additional coats
of clear dope and then do a fi nal sanding
with #400 grit no-load sandpaper.
The next step is to apply a coat of
the three-part automotive primer that
I wrote about in my May 2012 column
and proceed with the fi nish as described
in the series I wrote about using
DuPont’s ChromaBase paints.
The Crossfi re Extreme is now
receiving its fi nal trim color touches and
it should be ready for the top clear coat
in roughly four days. (I
am writing this in the
second week of June.)
I had hoped to have it
completed by now and
give you a full report
in this column, but you
know how it goes …
The great part about
this project is that I know
that the fi nished model
will fl y well. It did before
the refi nishing, and I’ve
done nothing during
the process that would
change that. Again, this
is my second refi nish
project, and I’d think long
and hard before doing
another one.
What tipped the scales in favor of
the attempt this time—and might
again in the future—is the fact that
there was no glow fuel residue to
contend with because the model is
electric powered. I was able to get the
model back to fresh balsa and apply a
fi nish that is lighter than the fi rst one.
I want to take this opportunity to
thank my friend, Sina Goudarzi, for his
help in coming up with a fresh, new
look regarding to a trim scheme. I’ve
taken a lot of ribbing throughout the
years for my signature Bronze Dog paint
scheme. This one is a vast departure
from that. Sina is an artist who helped
me to fl esh out an idea that I’ve had for
a new scheme.
In the next column I’m going to
expound on Sina’s talents and explain
how he might be able to help you with a
new trim scheme, too!
Another ChromaBase Update
I promised that I would continue
to pass along information on the
ChromaBase fi nishing process as I
became aware of it. I mentioned to my
friend, Dean Metzger, who is a manager/
technician at Klines Automotive in
Phillipsburg, New Jersey, how I was
using a silver blocking coat.
I told Dean that I was unhappy with
the bond to the sanded primer surface
of the model. He told me that DuPont
has a product that is designed to cover
the sanded primer surface and yield an
opaque blocking coat that also helps to
promote paint adhesion in subsequent
layers.
The product is called ChromaSeal
and is available in white (which I used),
medium gray, and dark gray. It is a
two-part product that must be used
in conjunction with an activator. The
activator is available in several heat
ranges; check with your dealer to be
certain which one to use.
The ChromaSeal covers almost
instantly, and you should only apply
enough to completely cover the
primed and sanded surface. After the
ChromaSeal cures, you will be able
to inspect the model’s surface for any
fl aws, dings, etc. You can then fi ll the
imperfections with spot putty, sand,
and then lightly recoat those areas with
ChromaSeal.
ChromaSeal has great fi lling properties
and it can be easily sanded with either
#400 no-load sandpaper or a Microfi ne
Scotch-Brite pad until the surface is
fl awless. I continue sanding to remove all
excess ChromaSeal, leaving the model
in one color, but with only a thin coat of
ChromaSeal left on its surface.
I found that a thin coat of white
ChromaBase paint was all that was
necessary to cover the ChromaSeal
and achieve a fully opaque surface. I
estimate that I used only one-half to
two-thirds the amount of paint on
the model as I did without using the
ChromaSeal beforehand. The net result
is a slightly lighter, but much better,
fi nish and better fl ow out for the paint
coats. I highly recommend it!
Till next time, fl y Stunt