Great Planes Piper J-3 Cub 20 Kit - GPMA0158 - Manual (Page 35)

Fill any scuffs and dings with balsa filler or by "expansion "
See Expert Tip below After the filler has hardened, sand
the entire structure with progressively finer grades of
sandpaper, ending with 320-grit
Many surface blemishes on a framed model are caused
by bumps and balsa chips on the work surface This
type of "ding" is best repaired by applying a drop or two
of window cleaner or tap water to the blemish, then
running a hot sealing iron over the spot to expand the
wood fibers After the surface has dried, sand the
expanded area smooth.
SPECIAL NOTE: Do not confuse this procedure with
"checking the C.G." or "balancing the airplane fore and
aft." That very important step will be covered later in
the manual.
Now that you have the basic airframe nearly completed,
this is a good time to balance the airplane laterally (side-
to-side) Here is how to do it
D 1 Temporarily attach the wing and engine (with muffler)
to the fuselage
D 2 With the wing level, lift the model by the engine
propeller shaft and the bottom of the rudder (this may
require two people). Do this several times.
D 3 If one wing consistently drops when you lift the model,
it means that side is heavy Balance the airplane by gluing
weight to the inside of the other wing tip Note: An airplane
that has been laterally balanced will track better in
loops and other maneuvers.
Cover the aircraft with Top Flite MonoKote film using
the sequence that follows Make sure the MonoKote film is
thoroughly stuck to the structure and all of the edges are
sealed Use a Top Flite MonoKote Hot Sock on your
covering iron to avoid scratching the MonoKote film.
You can practically eliminate wrinkles in your covering
that sometimes occur when the model is left out in the
sun or in the back of your car by following this
technique used in the Great Planes model shop:
A Cover your sealing iron with a Top Flite Hot Sock
and turn the heat about 3/4 of the way to the
high setting
B. Cut a piece of MonoKote film about 2" larger all
around than the surface you are covering Strip off the
backing and position the film Tack the film down
smack dao in the middle of the surface.
C Pull (as in stretch) the film toward one end, sealing
it to the balsa from the center out to the tip. Work out
any wrinkles and air pockets as you proceed with a
combination of circular and back and forth motion.
D Do the same procedure working the opposite
direction from the center
E Pull and seal diagonally toward the four corners,
always starting from the center The trick is to shrink
out any wrinkles before you seal the film to the surface.
F Use a heat gun to heat and to stretch the film around
curved surfaces like the stab and rudder tips, while
pulling on the excess material You may need to pull
hard to get out all of the wrinkles, so wear a glove if
you need to Follow-up the heat gun with your sealing
iron to secure the bond.
The idea behind this approach (which can be applied
to any part of the model) is pre-stretch the MonoKote
film as it's applied, and remove the air pockets that can
expand later to cause the sags and wrinkles.
The Cub 20 does not require much painting to obtain
the scheme shown on the box, as most of the finish is
done with Top Flite® MonoKote® covering The only
painting required is the cowl, windshield frame, dummy
engine and hubcaps
The technique we will describe here is how the model
pictured on the box was finished Remove the engine and
engine mount, landing gear, windshield and control horns
Make sure the structure is smoothly sanded with 320-grit
sandpaper. Remove all dust from the structure with a Top
Flite Tack Cloth so the covering will stick well
When covering areas that involve sharp junctions, like
the tail section, cut narrow strips (1/4" to 3/8") and apply
them in the corners before covering the major surfaces
The larger pieces of MonoKote film will overlap and capture
these smaller pieces This technique also bypasses the
need to cut the MonoKote film in these areas after it has
been applied DO NOT, under any circumstances,
attempt to cut the covering material after it has been
applied to the fin and stab, except around the leading
and trailing edges and the tip. Modelers who do this often
cut through the covering and part-way into the balsa stab
This can weaken the stab to the point where it may fail
in flight'
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