When to rebuild - 2012/07

In my past three columns, I have reported on my experiences with basecoat/
clearcoat finishing systems, materials, and techniques. I promised to get on with
other subjects this time, but it seems that I’ve hit a nerve with many.
I have received perhaps more mail, email, telephone calls, and comments from
friends on this subject than I have for nearly anything that I’ve covered in the past. I
have learned more since I wrote the last column, so I’m going to share that with you.
In my May 2012 column, I wrote about shooting on a coat of ChromaBase silver
paint as a blocking coat to cover the carbon-mat and filler coat spots that are left
after sanding. The basecoat easily goes over the silver in one thin coat, and at that
point you can begin trimming.
Since my last column, I have finished my Gene Shaffer-designed model that I
affectionately call the Oosa-Amma. (It was so named because Gene put a huge USA
on one wing and an equally large AMA on the other wing. Together it just seemed to
spell something that sounded like Oosa-Amma.)
I miscalculated the amount of “grip” that the ChromaBase silver paint would have
compared with the silver dope that I used for this purpose in the past. The silver
dope didn’t bond sufficiently to the filler coat, and the tape I used to mask off the
color trim areas would occasionally pull up some of the substrate. It was a pain!
Unfortunately, although the basecoat/clearcoat paints seem to grip better than the
dope did, I still experienced a few areas where very small amounts of the basecoat
would come up with the tape when it was removed.
I told my good friend and building/flying partner, Buddy Wieder, about this
problem during the trimming process. He told me that to prevent this from
happening he uses a low-temperature heat gun and flicks the area ahead of the tape
with some hot air as he removes it. I tried that and it worked perfectly!
The heat softened the adhesive on the tape just enough to make it release easily
from the surface, and no additional areas
of basecoat were pulled up throughout
the rest of the trimming process.
Because of this, I’ll continue using
the ChromaBase silver as a blocking/
indicator coat on future projects.
When to Rebuild and Refinish
I’m going to switch gears and discuss
repairing and refinishing. At last year’s
Nats, I had the unfortunate experience
of watching my new Crossfire Extreme
get picked up by a rogue dust devil and
slammed into the ground. The fuselage
was broken all the way around, slightly
behind the wing.
With help from many friends, I was
able to make some quick repairs and get
the aircraft flying again so that I could
complete the Nats program. I missed
making the Top-Five fly-off by a quarter
of a point!
Clearly the repair was a success and
the Crossfire was still properly aligned,
but the fix rendered the surface finish a
mess in the break area. There was some
cosmetic damage to the wingtips, but
otherwise the model was intact.
Additionally, a week before leaving
for the Nats, one of the wheel pants
shattered during the takeoff run on a
practice flight and two large pieces of
the pant pierced the bottom of the wing
and one went on through the top of
the wing and splintered the sheeting. I
was able to repair the damage quickly
because of the wonderful blending
properties of the ChromaBase paint
and clearcoat, but you could see slight
evidence of the damage from the right
sight angle.
The decision to refinish a model is a
difficult one to make. Many feel that it
is wiser to build a new airplane because
any good refinishing job will take
roughly as many hours to do as a new
build. I’ve done one refinish project on
a CL Stunt model and done properly,
it takes many hours. Ironically, that was
done on my 1987 vintage Crossfire and
it took many hours to complete.
What factors should you consider
when contemplating a refinishing
project? The most important is the way
the model flies. If you have a favorite
model that turns equally in both
directions, grooves in both upright and
inverted flight, and feels “good” to you
when it is at the end of the lines, that
is a prime candidate. That means that
the wing and stabilizer were installed
properly and that there is no decalage
between the two. That’s the most
important thing to get right when
building a Stunt model.
That’s how I felt about my 1987
Crossfire, and that’s how I feel about
my new Crossfire Extreme. In both
cases there was much rebuilding and/
or repairing to do along with the
refinishing. You must decide if the
damage that needs repairing—or any
alterations you might be considering—
will add extra weight that might render
the model useless afterward.
In the case of the 1987 Crossfire,
I decided to remove and replace the
movable surfaces with lighter ones and
to replace the bottom block for aesthetic
reasons. That model had too much paint
applied initially, and I thought that with
the modifications and a lighter finish, the
result would be a much lighter aircraft.
That worked out well; I was able to
remove 8 ounces from the airplane and
still obtain a great finish. The Crossfire
sat on the front row of the Appearance
Point judging at the 1990 Nats. It flew
much better than it had before, and it
looked nicer, too. A successful project!
The new Crossfire Extreme had one
huge additional thing going for it when
I decided to rebuild and refinish it—
because it is an electric-powered model,
there would be no fuel soakage to deal
with!
The Crossfire Extreme had a few
other advantages. It was built using
molded fuselage top and bottom shells,
unlike the original Crossfire, which
had top and bottom blocks that were
carved from solid balsa blocks and then
hollowed. I could easily mold perfectly
accurate new shells for the Crossfire
Extreme and replace the ones that were
damaged in the accident.
I had been using the Crossfire
Extreme as a test-bed model for some
possible modifications to the design,
which I would incorporate into the two
new ones that I’m building.
Paul Walker had flown the Crossfire
Extreme after the Nats last year and
he liked it, but he mentioned that he
thought it had too much flap area. He
suggested trimming roughly 1/4 inch off
the chord of each flap. I did that and
found that the model turned and locked
better, and even grooved better in the
round maneuvers. Thanks, Paul!
Refinishing flaps and/or elevators that
are attached to the model is too much
of a pain to even consider, so I removed
them, being careful to not damage the
hinge mounts in the fi xed surfaces. I had
replaced elevators on models in the past
and found that I could reuse the original
hinges.
During this process, I found out
something interesting. I had used Pacer
Formula 560 Canopy Glue to install
the hinges. I was moderately concerned
about the long-term strength of the
adhesive. It was diffi cult to trim the
wood and glue from the surface of the
large Great Planes hinges that I used.
The glue was well attached and even
remained slightly fl exible. There was no
danger of any of the hinges loosening or
pulling out. I will use this glue for the
hinge installation from now on!
I decided to remove all of the hinges
from the fi xed surfaces and install
new ones. It was too diffi cult to sand
away the catalyzed clearcoat and the
ChromaBase paint from the perimeter
of each of the 18 hinges!
I opted to cut the hinges out of the
fi xed surfaces by making cuts around
each hinge and removing the resulting
rectangle of wood that contained the
hinge. I cleaned up the slots to accept
balsa plugs.
After the plugs were glued in place
and sanded fl ush with the surface, I
slotted them and installed new hinges
in both the fi xed and movable surfaces.
This turned out to be easier and neater
and I remembered that this is what I had
done when I refi nished the Crossfi re in
1987.
Removing the catalyzed polyurethane
clearcoat, the base color coat, and
the trim colors is probably the most
daunting task when refi nishing. You
could patiently block sand all those coats
off the airframe, but that would take
hours … maybe days or weeks! I prefer
to use a paint stripper in a controlled
manner.
I used Klean-Strip KS-3 Premium
Stripper that is formulated to remove
epoxy and polyurethane-based paint. I
didn’t want to strip the fi nish off all the
way down to the carbon-mat layer. I
aimed to get down to the blocking coat.
With some careful experimenting, I was
able to achieve this goal and removed
the majority of the paint finish in a short
amount of time.
The stripper’s instructions say to
neutralize the residue stripper using lowodor
mineral spirits, which worked well.
A note of caution here: follow the safety
directions on the can exactly! Wear
splatter-proof eyewear and rubber gloves
while working with this stuff.
I was able to carefully block sand the
remaining paint and blocking coat from
the model using #220 grit sandpaper and
a small, flat block. After the paint and
blocking coat had been removed, I finesanded
the surfaces with #400 wet or
dry sandpaper and a foam sanding block.
I stripped and sanded the fuselage
first and then installed the new molded
shells before stripping and sanding the
flying surfaces. I nearly opted to forgo
installing the new shells and just refinish
the fuselage over the external repairs we
made at the Nats.
It was a good thing I didn’t! There
was some significant damage inside
the fuselage, and removing the shells
gave me the opportunity to brace the
damaged fuselage sides properly. I’m not
sure the model would have lasted long
with that sort of damage.
To ensure that the wing/stabilizer
alignment was maintained during the
repair process, I replaced one shell at a
time. The entire process—removing the
shells, fixing the damage, and replacing
the shells with new ones—took only two
days. I’m glad I did it!
I took the opportunity to make a
few aesthetic changes to the Crossfire
Extreme. I lowered the fin/rudder height
slightly and reshaped the ventral fin. My
projections are for a model that weighs
a couple of ounces less than it did with
the original finish. That is because of my
increasing familiarity with the basecoat/
clearcoat system.
Next time, I should be able to report
on the finished—uh, refinished—
product!