SPRAYING THE CHEMICALS
By Ron Alexander
the May issue of Custom Planes, we
discussed spraying techniques, equipment, etc.
In our ongoing series on covering using nitrate
and butyrate dopes, our next step is to actually
spray on the dopes. When you are covering an
airplane with fabric, most of the work is done
when you finally get to the spraying stage. You
will already have all of the fabric in place,
the rib-lacing completed, all of the tapes,
drain grommets, etc. will have been installed,
so you are ready for the fun -- the actual spraying
as a quick review, ensure that you have the
proper equipment, a good place to spray, and
that you choose the right day to begin the
process. Unless you have a climate controlled
spray booth -- doubtful -- then the perfect spraying
day would have a temperature of about 75 degrees
with relative humidity of 30% or less. We all
know these days are hard to find. With that in
mind, do not spray butyrate dope in temperatures
below 65 degrees or above 85 degrees. Also the
relative humidity should be less than 50% if at
all possible but 70% is acceptable.
not spray in direct sunlight or in the wind.
Try to make up a small spray booth as we
discussed in the last article. It will save you
from a lot of problems. Don't forget to wear
the respirator and to use the spraying
techniques we discussed.
SPRAYING BUTYRATE DOPE
all of the tapes, etc. are in place, you will
begin to spray coats of non-tautening butyrate
dope. Before you begin to spray heat up your
small iron to 225 degrees F and carefully iron
down all pinked edges of finishing tapes. This
will ensure a nice, smooth surface.
will want to spray on 3 cross coats (a cross
coat is one pass with the spray gun north and
south followed by another pass east and west) of
non-tautening butyrate dope. The idea is to
achieve a smooth, plastic finish. You will need
to thin the butyrate dope with butyrate
thinner. The ratio should be approximately 1
part of thinner to 1 part of butyrate dope. If
the relative humidity is high (above 50%) and/or
the temperature is high, you may have to add a
retarder. This mixture will usually be about 1
part retarder to 3 parts of reducer. Lets take
a look at what a retarder actually does and why
you may need it.
dope dries, the rapidly evaporating solvents
actually lower the temperature at the surface of
the object being sprayed. Any water vapor in
the surrounding air will then condense on the
surface. If the humidity is 80% or more this
condensed water vapor will give the dope a milky
appearance. In the fabric covering world this
is known as "blushing". Any blushing that
occurs with dope will actually weaken the dope
itself and its bond. All blushed dope must
be sanded off and the area resprayed. With this
in mind, you can easily determine that you do
not want blushing to happen. Remember, this
will usually only occur when you are spraying in
a humid environment. You have two choices:
wait until a better day or use the chemical
known as "retarder".
retarder is a special solvent that will slow the
drying process of the dope, thereby minimizing
the chance of blush. It is mixed in with the
butyrate dope and thinner prior to spraying.
Retarders will not solve all of your problems.
If you elect to spray on a 95-degree day with
the humidity at 99% retarders will not prevent
blushing. Wait for a good day. Another tip, do
not wet the floor of the area where you are
spraying just prior to beginning the
application. If you do this the humidity will
increase. If you need to wash the floor let it
thoroughly dry before you begin to spray.
Finally, use only the recommended amount of
retarder. Use of excessive retarder can cause
return to our spraying process. Prior to
spraying on the first coat of thinned butyrate,
thoroughly clean the surface using butyrate
thinner and a clean rag. Use only cotton type
rags. Do not use any rag that may have any oils
or other contaminants. After this step, just
prior to spraying wipe the area with a
commercial tack cloth. This will remove any
lint, dust, etc.
First Coat of Butyrate
on a full cross coat (2 coats) of thinned,
non-tautening butyrate dope. If you need to use
a blush retarder add it as you are thinning the
mixture. Completely mix the solution and then
pour it through a paint filter into your spray
gun. Spray the mixture as we discussed in the
last issue. Spraying dope is fairly easy due to
its consistency. It is somewhat thick so it
will not have a tendency to run or sag unless
you use the wrong techniques. After you have
sprayed on the first coat make another pass with
the spray gun in the other direction to complete
your cross coat. Remember to always begin this
process on a small surface such as a control
surface. This will allow you to practice and
not make a mistake on something large.
Second Coat of Butyrate
the first coat of butyrate dope has dried for at
least 1 hour (this will depend upon
temperature), spray on another coat of butyrate
dope. Again, allow this coat to dry for at
least 1 hour and then apply a third coat.
will now have 3 full cross coats of butyrate
dope on the surface. The fabric should have a
smooth plastic appearance.
applying the third coat of dope, it is time for
our favorite activity, sanding. You should use
320 grit wet or dry sandpaper and thoroughly
wet sand the entire surface. Do not sand
over rib laces, rivet heads, or over anything
that might cut through the fabric. Use of a
sanding block is very helpful. As you wet sand
use a hose to wash off the residue. Do not
leave any residue on the surface, as it will
prevent additional coats from bonding properly.
The area you have sanded should feel very smooth
as you run your hand over it. You will be able
to detect areas that need more attention with
you have sanded the entire surface, set the part
aside and let it dry completely before you spray
on any additional coats.
people will apply additional coats of butyrate
dope for a smoother finish. It is not necessary
to apply more than 3 cross coats. However, if
you desire a really smooth surface you may apply
2-3 more coats after sanding.
Silver Butyrate Coats
Polyester fabric, like most fabrics, will
deteriorate in sunlight if not properly
protected. This deterioration will occur within
a very short time -- 6 months or less, without
protection. All types of fabric will
deteriorate in direct sunlight unless
protected. Polyester fabric is not as
susceptible to this problem as cotton; however,
if bare polyester fabric is left in direct
sunlight for 12 months it will lose over 85% of
its strength. Cotton fabric exposed to the same
sunlight for the same period of time will
deteriorate almost completely. Polyester fabric
is protected from the UV rays of the sun by
applying chemical coatings containing aluminum
pigment. Application of the recommended number
of aluminum coats will provide adequate
protection for years. These coats are termed
"silver coats" even though the actual mixture
itself consists of an aluminum pigment mixed
with butyrate dope.
Randolph's Rand-O-Fill is a pre-mixed butyrate
dope that contains the proper amount of aluminum
pigment. The aluminum pigment has a tendency to
settle to the bottom of the container. It is
absolutely imperative that you mix the pigments
into the solution just prior to spraying.
Failure to do so will leave a mixture that will
not adequately protect the fabric. Often a
5-gallon container will have collected most of
the pigment in the bottom just from settling.
You will find the pigment has settled to the
bottom of the can to the extent that it feels
like a hard layer. You must use a stirring
stick or some means to break that loose and mix
it throughout the can. Again, I want to
emphasize that this must be done just prior to
The aluminum pigment is heavy and will settle to
you desire, you can purchase aluminum powder in
a can and mix it according to directions into
butyrate dope thus making your own mixture for
the silver coats.
silver butyrate dope should be thinned the same
as our clear coats -- one part of thinner to one
part of silver dope. If you need to retard the
mixture do so the same as previously discussed.
Silver dope is also subject to blushing.
should apply at least 3 cross coats of silver
onto the surface to provide adequate protection
from the UV rays of the sun. A quick check to
ensure the proper amount is in place can be done
by placing a 60 watt light bulb inside the
structure and make sure no light is visible on
the outside. (Use a protected bulb that will
not shatter and ignite the solvents inside the
sand after each coat of silver. Allow the
surface to completely dry before you begin to
wet sand. After sanding allow the water to dry
before applying the next coat. Always clean the
surface and use a tack cloth just before
spraying on a coat.
Additional coats of silver may be applied for a
smoother finish. Sand after every other coat
using progressively less coarse sandpaper. In
other words, begin with 320 and then go to 400
or more on succeeding sandings.