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If you are
a beginner looking for advice on how to tackle a good old-fashioned
dope finish on your model, this article from the March-April edition
of American Modeler is about as fundamental as it get. The author
recommends methods for operating and cleaning a spray gun, how to
properly prepare a model for painting, taping off trim lines, and
even achieving that high gloss finish that so many contest grade
models have. If you are looking for pictures, then this article
is not for you - it's all business. However, if a picture is worth
a thousand words, then the inverse must be true that a thousand
words is worth a picture. That being so, this article contains three
pictures.
Scalecraft & Finish
By Paul Plecan
You want your next scale job to be slicker, smoother, and shinier,
right? It takes patience and requires effort.
In this review we will cover Ukie, R/C and F/F in that order.
Since control-line weight is not an overwhelming consideration,
we can duplicate the "average" finish seen at most contests. Most
of us may be familiar with the amateur approach - too many coats
of dope too hastily applied, insufficient drying time and improper
sanding between. Yes, there is much more of an art to "sandpapering"
than meets the eye. It quite often is the hardest skill to master.
For the usual glow-plug equipped model, we need a fuel-proof
type undercoating and pigmented dope. Naturally, there are favorites,
but this is a personal thing. What works best for you can only be
determined by Y-O-U.
Some brands can be mixed, but formulas frequently vary and mixing
of two brands can result in a useless goo. So test first before
trying unknown combos on a model. The beginner should go the whole
route with a single brand. Only deviation we make from the one-brand
rule is to use inexpensive thinner for cleaning hands after working
with dope or lacquer. While cheap thinner will do for this do not
use it with the undercoating or pigmented dope.
To apply the dope to the model, we need brushes. Camel-hair,
ox-hair, and sable brushes are long-time favorites - you will get
the quality you pay for. Cheap brushes shed bristles and soon come
apart entirely. A wider brush will speed up work; 1/2" wide brushes
are the minimum to consider, 3/4" and 1" widths are popular. At
least four brushes are needed: one for undercoating, the others
for light, medium, and dark work (you discover that a brush, once
used with black, can not do much of a white dope job). Between two
and three dollars is needed for the four brushes, so take care of
them.
Never, never allow dope to dry on a brush. Any time you must
lay it down, dip it in thinner and lightly squeeze out the dope.
A handy way to clean brushes is to have three small (approx. 1-oz)
jars lined up, 2/3 full of thinner. Dip the brush in #1, squeeze
dry with clean rag, repeat dip-dry sequence with #2 and #3. Keep
jars lightly covered, replenish thinner when dirty. Tighten caps
when not in use to keep thinner from evaporating. To get quality
brushes you may have to go to an art supply store.
An alternate method for applying dope is with a spray gun. A
unit rated at a minimum of 3 cubic feet of air per minute is required.
Forty or fifty dollars for a new outfit.
Happily, there is an economical alternative in the spraying department.
You can get spray cans that are easy to use, or attachments that
have separate air pressure cans and dope jars. The latter allow
custom-mixing of colors.
For two to three dollars, you get a head assembly, a 4-oz jar,
and a 15-oz pressure unit (extra pressure units are between one
and two dollars). Each pressure unit can spread almost a quart of
dope. It will pay you to have separate units for light and dark
work, maybe a third unit for intermediate shades. The reason? It
takes a fair bit of thinner through the sprayer to clear out the
previous color. Two or three "rinsings" to go from a black to white
for instance. There are instructions on the label - read and follow
them! When liquid in any integral spray can unit is depleted, always
release remaining gas pressure.
Another item on the shopping list is sandpaper. "Bargain basement"
paper will not do. At least 6 sheets each of 260 grit and 320 grit
waterproof sandpaper. You may find grit designation will vary -
280 and 400 grit is just as good as grades listed above. If sheet
size is about 9" x 11", it will be much handier if these are cut
into quarters or sixths.
Let's assume your model is fully assembled, all fabric or tissue
surfaces having been water-sprayed and dried to tighten up covering.
Before we start with the undercoating, clear dope all fabric or
tissue areas. Undercoating directly on fabric would overly increase
weight.
You apply undercoating with a brush or smooth it out with your
fingertips, making it sink into the pores of the balsa. This is
our main objective - to fill in the pores.
Allow the first coat 30 minutes to dry, then very lightly skimming
the surface, sandpaper away the major imperfections. This first
sanding can be done with plain sandpaper of garnet type, no rougher
than 220 grit or 6/0. Your thumb and forefinger keep the paper from
slipping away, the pressure on the work being exerted by the four
finger tips. Lightly, now. Best motion pattern is circular or elliptical.
This eliminates the sudden stops of back and forth straight motion,
where the sandpaper digs in at the ends of the stroke. Develop an
even pressure and never linger in one spot - idea is to remove an
equal amount of undercoating all over the model.
In case anyone is confused, the term "undercoat" here means any
of the items known as primer surfacer, primer sealer, sanding sealer
or balsa fillercoat. A second coat of undercoating applied is allowed
40 minutes to dry before sanding. Use the 260/280 grit waterproof
paper from now on - whether you dip it in water or use it dry is
not too important. Damp or wet usage is preferred. With the application
of the third coat of undercoating, it is a good rule to allow even
more drying time ... 50 minutes or more is a necessity. From this
point on, use the paper thoroughly wet. A dish partly filled with
water should be near at hand. Dip the paper into it as soon as you
feel it "drag" when sanding. You, should find the balsa-dust and
talc floating away from the paper each time you dip it in the water.
If it tends to clog up and lose its "bite," then the finish has
not been allowed sufficient time to dry. Butyrate type dope seems
to take longer to dry thoroughly and tends to clog the sandpaper
quicker, but remains more flexible, even over a long period of time,
resulting in less "crazing" or splitting. This probably is due to
the use of more plasticizer (castor oil, etc.) in the solution.
The sanding of the third coat should continue until you can see
that most of the surface coating has been removed and you are sanding
the model again. But there will be a great difference now - if you
hold the model near a strong light and look lengthwise at any area,
its surfaces should be quite slick. There is a chance that the pores
of the wood can still be detected. If a top-grade finish is desired,
another coat is called for (plus sanding).
When "three or four" coats of undercoating are specified for
a particular airplane, we must remember the biggest "variable" of
all is the model-builder. One reads the can labels, shakes and stirs
the solution well. Another just zips along, paying slight attention
to article, label, or well-meaning friends. This is the guy who
needs the extra coat. Maybe more, if he applies it with a skimpy
type technique.
By this time you should be some sort of judge on drying times,
sandpaper pressure, number of coats. And your handiwork should reflect
more truth when the word "workmanship" is bandied about. Use of
waterproof sandpaper should not mean that the model stays wet continually.
Once sanded, wipe dry with a cloth or crumpled tissue. And even
though dope tends to sink in (undercoating, too) when the first
few coats are applied, you can sand too deep. If you do, this may
let a bit of water penetrate under the doped surface. You'll know
when it happens, as just a little water in the balsa wood will cause
a blister or puffing up to occur right where the water seeps through.
Once this happens, there is almost nothing one can do to correct
it in a hurry. A series of punctures with a very sharp X-actro blade
or pin in the vicinity of the blister will aid in letting the moisture
escape, as will leaving the model in any well-heated dry area. About
a foot away from a furnace, or even farther away if placed near
a stove. Check the damage within 20 minutes, as you may place the
model too close and the finish may blister from heat. Safer to let
it dry overnight, not too close to any very hot surface.
You should have a model now that looks great even without color.
But the pigments are what we are after in any paint job. Before
you proceed with the color dope here's a tip for the "all-out" slick
finish fans. To be doubly sure of smooth surfaces, a coat of silver
dope at this point will reveal all the dimples, low points, slight
pore effect or other blemishes. Nothing else will more critically
highlight the slightest imperfections ... unless you want to use
a microscope!
On with the colors. Always apply colored dope in smooth strokes
of the brush, flowing the liquid on with a minimum of "stroking."
This minimizes streaking and gives more even coverage. Keep a well-thought-out
program in mind; start with nose, left side, then right side of
fuselage. Top left wing panel, then bottom. Repeat for right wing.
Repeat wing sequence for horizontal tail surfaces. Left, then right
sides of rudder; lay aside to dry. Somewhere along the way now you
will realize that each succeeding coat of dope traps all the rest
under it. If any previous phase was rushed through, the dope remains
"wet" in actual truth even overnight. It is for this reason that
overnight drying is desired. The sanding goes that much easier,
and less clogging takes place too. You can almost pose as an expert
now.
Certain colors have more "body" and hiding power than others.
Well-mixed black dope will cover most any previous color in two
coats. White takes more than six coats to effectively cover black.
So it depends on the particular color scheme as to how many coats
are required. If you choose a model with a light-colored paint job,
you will find that it takes four coats of light yellow or white
to achieve a solid body of color, without faint streaks or blotches.
Dark stripes or other trim should effectively cover the light areas
with two coats - three at most. If you aren't getting effective
coverage, you've let the dope settle too long after stirring or
shaking. You'll hardly ever need to change brands, as a poor paint
job reflects mostly the model-builder's lack of "know-how" more
than the quality of the dope used. There is a limit as to how much
you can learn by the "book" - you just have to get your hands dirty.
Which brings us to a minor matter - clothing. Cultivate the habit
of wearing your old tattered shirts and slacks when painting. It
only takes a drop or two to mess up some of your favorite threads.
And getting the pigment entirely out is only a matter of opinion
(you'll wind up with some sort of spot in any event). We don't usually
notice the spot until the dope has dried to some extent - it's too
late then. Shoes can be a problem too, but if you keep them shined,
the wax generally keeps the dope from actually getting into the
leather. Note that we said "generally."
All light colors go on first. With white, six coats are needed.
Most other light colors. four coats. Dark covering coats, three.
In extreme cases; you could add two additional coats to each of
above.
The use of rubbing compounds is recommended on the "museum piece,"
U-control stunt or scale or an R/C model that you have invested
in heavily, money-wise. Along with the rubbing compound, you'll
want to use gloss wax. Read the instructions on the can and use
plenty of elbow-grease.
After flying, the sooner you clean your model, and the less time
you leave spilled fuel on your model, the better. Pactra's Plane
Kleener or rubbing alcohol will cut into the exhaust sludge that
streaks the engine cowl and the bottom of the fuselage. Store models
in an unheated area of the house, as the average "indoor" portion
of most houses is too dry, causing the finish to "craze" in short
irregular cracks that generally cover the entire model. With the
passage of time, cracks sometimes enlarge so the darker areas split
enough for the lighter coats to be seen.
This condition can be prevented on your next painting job by
adding a drop of castor oil to each ounce of dope that is used.
Castor oil slows down the drying of the dope, calling for a longer
wait before sanding. With even an overnight drying period. you will
find the sandpaper loading up more more quickly (so you will need
about double the usual requirements).
The model-builder who uses a spray gun should follow manufacturer's
instructions - and always keep the gun in motion. Maintain an even
rate of rotary motion for even coverage, working at a constant distance
from the painted surface. You soon learn the vital importance of
snug masking and air-tight "cocooning" of lighter areas when working
with dark colors. The merest hole will allow some of the darker
mist to penetrate to mess up the job.
Unless you have a large work area that is both heated and well
ventilated, forget spraying. A warm area (65 to 70°) is needed to
get proper atomization and ventilation (through-wall fan duct) is
needed to carry off fumes. Without proper ventilation, an average
spray session will leave droplets all over you and everything in
the spray room. An open door to the rest of the house will mean
a dope-mist on drapes and furniture. Always have all the preparatory
work finished before loading your spray gun. You don't want the
pigments to settle before you get into action. By loading and being
ready to spray, you get those pigment particles onto the model,
where they belong.
For R/C .models, the same general instructions apply. Do not
overload such planes with too much pigmented dope. You already have
enough payload in the form of R/C equipment. And on the larger models,
weight builds up very rapidly. A model doubled in size has four
times the area to paint ... tripling the size multiplies the surface
area nine times. Get the dope onto the model right after the pigments
have been agitated into suspension; allowing them to settle calls
for extra coats that otherwise would not be needed.
For free-flight scale, the old style colored tissue model with
tissue trim or numbers cut from contrasting colored tissue and doped
on appears dead as a dodo. A full paint job is the order of the
day, with flying performance suffering appreciably. Lucky for us,
the engine makers have not been sleeping; we can still haul the
deluxe paint jobs around effectively. Due to the greater shock hazards
inherent in F/F, it is recommended that you plasticize the dope
even more than for Ukie jobs. Two drops of castor oil per ounce
of pigmented dope for tissue-covered models. So on the flights where
the model whacks a pole, tree-trunk or brick wall there is less
chance of splits in the tissue covering. Silk, being more resistant,
will not need heavily plasticized dope ... no more than 1 drop per
ounce.
Weight being an important factor in F/F scale, you want the maximum
pigment coverage with the minimum number of coats. Since it takes
so many coats of white or other very light shades to "cover," stay
away if possible from designs that call for light paint schemes.
Good pigment coverage is had with red, blue, olive drab and other
middling-to-dark shades. Silver covers very well in just one or
two coats. But it accentuates every wrinkle and scratch in the surface
of the model.
How to pull masking tape away from a paint job without tearing
the covering or "lifting" the edge of the paint job? When using
a brush to apply pigmented dope along the edge of masking tape,
try to avoid painting the tape. Paint only to the edge if you can.
Naturally, you will "miss" some of the time. Second, don't overdo
the number-of-coats bit. With two dozen coats, you've practically
bonded the tape to the model. On many coats, peel off early, when
dope is comparatively pliable. On few coats, peel late, dope can
be quite dry. Pull tape off with it doubled back running along on
itself 180 degrees. This gives the maximum shearing action where
the tape is parting company with the model. And pull in a slightly
side-wise manner, away from the doped area. The worst you can do
is pull at right angles to the work surface - then you are trying
to peel the tissue off too, to some extent.
To mask off long sweeping curves, a template is needed, to be
sure that the curve is the same on both the left and right sides
of the model. On intricate or small lettering or trim jobs a steady
hand is called for.
Where lettering or trim exceeds 1/4 to 1/2", a patch of masking
tape is laid on glass, the design then cut into the masking tape
with a frisket knife with point razor-sharp. Granted, it is a major
problem just lifting the masking tape "stencil," aside from properly
positioning it on the model. But the results are worth the effort.
It is the only way a design can be painted on a concave or convex
area. A decal will not lay flat in such an application. On flat
surfaces, there is no problem. Fuel proof decal sheets are available
in almost any color and can be cut to almost any required shape.
Work with care. A poor construction job can not be hidden under
the best paint job. When the model is built well, when high quality
finishing ingredients are used, the result is a delight to the eye.
Posted December 30, 2012
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