PAINTING... Solving the Mystery
By Ron Alexander
Ask a group of airplane builders whether or not you should paint
your own airplane and then stand back and listen. The opinions will
equal the number of people present. The painting issue is very
important since the final finish of our aircraft is most of what others
see. When someone inspects your airplane they will also judge the
quality of construction by the final finish. The painting needs to be
done properly. If you have no painting experience you will usually
view this as a formidable task. It is very tempting to dig out the
yellow pages and begin looking for a professional painter.
Professional painters usually have one goal in mind, provide a quality
paint job as quickly as possible and then move on to the next job.
Professional painters are often not familiar with the type of paints
used in the sport aviation industry. This is particularly true with fabric
covering topcoats. Painters are professionals so they are expensive.
The cost of painting an airplane varies from one location to another
but it is not unusual to be charged $3,000-$4,000 or more for a
quality paint job. Professional painters have the advantage of a
properly equipped facility along with top of the line painting tools.
They also should be experienced with the preparation procedure and
the painting process. It is very tempting to fly your airplane in and
leave it with a pro. It is also noteworthy that hiring someone to paint
your airplane does not affect the major portion rule of custom
building. It is perfectly acceptable to contract the painting.
You can complete a professional paint job on your custom built
airplane even if you have no experience. Painting equipment and the
paints used today make it much easier for an individual to paint their
own aircraft. After all, you have spent years building this masterpiece
so why would you want to hire out the painting? The custom airplane
builder will spend more time and expend more effort in the painting
process than will a professional. It’s your airplane! You have built it
from the beginning with attention to every small detail. You want the
same attention on the final finish. Sure you don’t have a bug free
working environment like a professional shop and you may not have
the experience of a pro but all custom builders possess two major
advantages, the desire for perfection and the quality of
perseverance. With those two characteristics you are capable of a
professional looking paint job. Basically, all you need is proper
equipment, time, the ability to follow directions, and practice. Even if
you have never picked up a spray gun you can do the job.
A charcoal filtered respirator.
Now that I have persuaded you to paint your own airplane where do
you begin? You will need some sort of paint facility, good painting
equipment, and the knowledge necessary to prepare and paint the
airplane. I will discuss all of these in this and the next article. I will
discuss what type of facility is needed, spraying equipment, how to
prepare surfaces for paint, and the actual process of painting.
When should you paint your airplane assembled or prior to
assembly? A first time painter will be better served to paint the
airplane prior to assembly. It is much simpler to paint the individual
parts of an airplane rather than to paint the entire airplane at one
time. However, most of our airplanes are smaller in size so painting
the completed airplane is not as difficult as painting a large
assembled aircraft. In either case there are ways to accomplish a
quality finish that will be discussed later. Some builders will want to
wait until they have test flown the airplane prior to painting. That
enables them to correct problems and make necessary changes
resulting from the test flying period. Some of these changes could
affect the finish of the aircraft so they will wait until this time for final
painting. Again, if you possibly can, paint the airplane prior to
assembly. Painting an airplane is literally 90% preparation and 10%
painting. Be ready to spend most of your time preparing surfaces for
painting. The actual spraying process consumes a small amount of
time. Remember also to practice-practice-practice. Before you begin
applying the topcoats you will have acquired experience spraying
primers. Obviously, the primer coat does not show so if spraying
mistakes are made they can be easily corrected. You can also
practice spraying techniques on large pieces of cardboard, old doors,
masonite, stove pipe, etc.. If you are painting a fabric covered
airplane you will have literally hours of practice spraying the more
viscous coats of chemicals used on fabric. Then when you are ready
to spray the final color coats you can do so with confidence. The
bottom line is this painting an airplane is usually approached with
much more fear than is necessary. Whether or not you elect to paint
your entire airplane you certainly will end up painting a number of
small parts. Consider painting the airplane yourself. You can do it!
FACILITIES FOR PAINTING
Ideally you would have a clean, well lit, temperature controlled paint
shop. Realistically, you will probably have to paint in your garage.
Some builders are fortunate enough to locate a hangar or other such
facility for their painting. Bear in mind that some airports will no
longer allow painting unless the shop is designed and built to comply
with local restrictions. Certain locations require the user of solvent
based paints to filter overspray and have a method of collecting
waste. If you are using a water based paint you will not be faced
with this problem.
You do not want to use your basement for painting. The fumes will
permeate the entire house and you will not be popular with your
family. A garage or workshop is adequate. After you have found a
suitable location for painting the next step is to build a poor man's
paint booth. This can be done very easily by using PVC pipe and
plastic sheets. Build a square frame out of wood or PVC pipe large
enough to cover your airplane or the largest surface you will be
spraying. You should allow enough space to be able to walk around
the surface. You can hang the frame from your ceiling with pulleys so
you can raise and lower it. Cover the roof and sides with plastic
sheeting stapled or taped to the frame. Tape the sheets together
using duct tape. At one end of the booth place a furnace filter and at
the other end an exhaust fan. This will provide filtered air. Make sure
the fan has an enclosed motor with no chance of sparking. If you are
unsure about the fan, leave it out and quit spraying when the booth
becomes full of overspray. The overspray will settle in minutes and
then you can go back to work.
Lighting is of utmost importance. Your booth should have an
abundance of lights on moveable light stands or any other way you
can arrange them. Shield the bulbs with chicken wire to prevent
breaking which could ignite paint particles. When you are painting
you cannot have too much light. You will also want to hang small
pieces of your airplane for spraying. This can be accomplished by
using welding rod hooked over a length of pipe. Then attach the
small pieces to the welding rod. The length of pipe or wood can be
suspended from the top of the spray booth.
You do not want to paint in the following areas: (1) outside in fog or
high humidity, (2) outside in direct sunlight, (3) in the wind, (4) in a
dusty place, (5) in a rented or borrowed spray booth where you can't
take your time. You will usually not be able to paint when the
temperature is below 60 degrees F. When you have finished painting
you can throw away your paint booth or donate it to a friend. It is
important that you have a clean, well ventilated, and well-lit area.
Do not try to save money on painting equipment.
High quality paint rigs produce high quality
finishes. There are four basic methods of applying
paints. They are: (1) conventional pressure fed
gun, (2) High-Volume, Low-Pressure (HVLP)
systems, (3) airless spraying, and (4) electrostatic
spraying. Airless spraying and electrostatic
spraying are used primarily in production work.
Airless sprayers force paint through a small tip
opening at extremely high fluid pressures ( usually
1200 to 3600 psi ). Using this amount of pressure
affords airless sprayers the ability to spray almost
any coating. Because of the high pressures airless
sprayers are rarely used in the aircraft industry.
Electrostatic spray systems charge the paint at the
tip. A high voltage difference is set up between
the paint and the object to be painted. The charge
attracts the particles of paint and they wrap
around the object. This type of spraying is used in
Conventional Spray Systems
That leaves the custom aircraft builder with 2
choices of spraying equipment, conventional
pressure and HVLP. The conventional system uses
a high pressure source, usually an air compressor.
The air compressor must be capable of delivering
about 40-50 psi at the spray gun. This means the compressor must
have adequate storage and be capable of supplying 90 psi+ at the
compressor outlet. These high pressures push paint out of the gun
and mix it with the airstream. Because the pressures are so high the
paint is atomized effectively so that it can be applied to the surface.
High pressure guns have been around for years. They can be used
with large volume "pressure pots." Pressure pots hold up to 5 gallons
of material that is handy when applying doping chemicals used on
fabric airplanes. The spray guns used for smaller jobs are usually
suction fed. The same high pressure is used to atomize the paints.
The coating is held in a quart cup and it is drawn into the airstream
by the high pressure airflow creating a vacuum. If you are going to
use the pressure-fed gun or the suction gun buy a good brand. Make
sure the cup has a means of venting that will allow you to position it
horizontally without dripping paint on your surface. The nozzles used
with paint guns are as important as the gun itself. Certain nozzles are
to be used with specific types of coatings. You will need to purchase
the proper nozzle recommended by the manufacturer for the coating
you will be spraying. If you choose the conventional spraying method
be absolutely sure you have moisture traps and filters on your air
compressor. Another point, if you are using a pressure pot you must
keep the hoses clean. If they are not properly cleaned particles of old
paint will be loosened and sprayed onto your project. A disadvantage
of traditional high pressure painting is the high velocity of the paint
particles as they reach the surface. Many of the paint particles will
actually bounce off the painted surface and gather in the air in the
form of a cloud called overspray. Often a large percentage of the
paint itself (30-40%) which is used ends up as overspray on the floor
or drawn through the filter. At the price of paint that can amount to
a lot of money passing through a filter.
High Volume, Low Pressure Systems
Because of the waste and overspray problem, a new technology was
introduced: High-volume, Low-pressure (HVLP). The pressure used in
these systems to atomize the paint is much lower (3-5psi). The paint
cup is pressurized forcing material to the nozzle. As a result, the
overspray problem is minimized. Most of the paint actually adheres
to the surface being painted instead of bouncing off and collecting
elsewhere. To achieve this advantage a HVLP spray gun must be
machined to a higher degree than an ordinary spray gun. The HVLP
spray gun is designed to gather the air inside the gun and send it
downstream with as little pressure drop as possible. This means
careful machining must be carried out to reduce turbulence that
impedes airflow. In older pressure guns if a passage was too small
you simply increased the pressure and solved the problem. That
means conventional spray guns do not have to be machined to exact
Two types of HVLP spray systems are available. One is the turbine
system and the other is a conversion spray gun. A conversion spray
gun uses regular high pressure air and drops the pressure before it
goes into the gun. A conversion gun is simply a HVLP spray gun with
a regulator in its handle. This type of system requires a high capacity
air compressor with moisture traps. The HVLP type we will discuss is
the "turbine system." An air compressor is not used with the HVLP
system. Instead, the system is equipped with a turbine not unlike
your vacuum cleaner. In fact, years ago Electrolux supplied a sprayer
attachment with their vacuum cleaner. Evidently that was the first
HVLP system. The turbine need only supply a large volume of air to
the gun to work effectively, high pressure is not necessary. All that is
needed is an air source that emits about 50 cubic feet per minute of
air at less than 10 psi that is sufficient to power the HVLP spray gun.
A turbine is a high-speed centrifugal blower motor operating near
20,000 RPM. Because of the high RPM the air is heated due to the
friction. This heat serves to eliminate all moisture from the atomizing
air. This heating effect also reduces the flash or drying time that
helps to minimize blushing. A HVLP system is sold with a turbine,
hose, and spray gun. The majority of the cost is found in the spray
gun and not the turbine due to the machining requirements of the
gun discussed earlier.
HVLP spray guns also are either non-bleeder or bleeder. The term
non-bleeder means that when the user releases the trigger of the
gun the air flow stops. With a bleeder gun the air flows continuously;
the trigger only starts and stops the flow of paint. A bleeder gun will
provide a better finish. However, use of a bleeder gun requires care
because dirt and other objects can be easily blown onto a painted
Lets look at the advantages of the HVLP system. First of all, the
overspray is reduced dramatically. That means a saving in coatings
typically of about 30-40%. Secondly, an air compressor is not
needed. The units come with a 110 volt turbine that can be used
anywhere. Thirdly, the moisture problem is eliminated. One of the
most significant problems a painter faces is the presence of moisture
that can splatter onto a surface along with the paint. The heating of
the turbine introduces heat that eliminates up to 95% of the
moisture content. Lastly, HVLP systems enable the beginning painter
to achieve a professional looking finish. They are easier to use than
conventional spray outfits. Individuals who have experience painting
with conventional spray outfits will have to learn the different
techniques required to use HVLP systems. HVLP systems expend
heavier amounts of material than conventional guns. This, of course,
means different techniques. Remote paint pots with capacities of 2.5
gallons are also available with these systems.
The Axis HVLP paint system.
To conclude, HVLP paint systems have obvious advantages. Two
different brands are shown in the article. The price of these units is
usually between $700-$800. That includes the turbine, 30 feet of
hose, and the spray gun. You will also want to purchase additional
nozzles for the spray gun depending upon the type of paint you are
spraying. Many people recommend an additional length of hose that
reduces the temperature of the air reaching the spray gun. If you
have a conventional system that is high quality then by all means use
it. Professional painters have been using conventional systems for
years with excellent results.
Certain health hazards do exist with spray painting. Of course, the
hazards depend entirely upon the chemical that is being sprayed.
The most significant health hazard occurs when atomized chemical
particles are inhaled. You must protect yourself with an adequate
respirator. A charcoal filtered respirator, such as the one pictured, is
sufficient for most primers, dopes, and paints. However, if you are
using any type of polyurethane paint you should have a forced air
breathing system. Polyurethane paints emit polyisocyanides that can
be extremely hazardous to certain individuals. Some people have
severe reactions to polyurethanes so don't take a chance. A simple
forced air breathing system is pictured and is manufactured by
HobbyAir. It is available for less than $400. A good investment to
protect your health.
You also need to protect your skin. Wear Invisible Gloves barrier
cream or latex gloves when mixing or spraying. I would also
recommend you purchase a Tyvek spraying suit. They are not very
expensive and they will protect both your skin and your clothes. If
you do not use one of these suits wear long sleeves and long pants.
If you spill solvents on yourself, remove your clothes and wash the
area with soap and water and put on fresh clothes. Do not mix paints
with an electric drill. The motor could spark and cause a fire. Use eye
protection in the form of goggles when mixing and spraying. Keep an
eye wash station nearby in case of emergency. You should also have
fire extinguishers handy that are rated for petroleum fires. Under
certain atmospheric conditions the action of sanding or spraying can
generate static electricity. When this static charge is transferred to a
surface the resulting spark could ignite solvent vapors. Ground the
structures being sanded or sprayed.
Now that we have determined that we can paint our own airplane,
we know what facilities are required, and we understand the
equipment, our discussion will be how to prepare different types of
surfaces and followed by a discussion of the actual spray painting
Painting an airplane to achieve an award winning finish can present a
challenge. In part 1 of this article, I discussed the fact that you can
paint an airplane yourself with quality results. The spraying
equipment, facility needed, and safety aspects were presented in
that article. This part deals with the preparation of surfaces and the
basics of the painting process.
As previously mentioned, preparation demands 90% of the time
involved in painting your own airplane. Proper treatment of the
component parts is both time consuming and essential. The most
common surfaces encountered in aircraft painting are aluminum,
steel, wood, and fiberglass.
PREPARATION OF SURFACES
Aluminum surfaces are treated differently depending upon whether
the metal is new or used. Paint must be able to "grip" or adhere to
the surface onto which it is applied. Most aluminum surfaces have a
layer of pure aluminum on the surface called alclad that protects the
metal from corrosion. It is very smooth and not favorable to paint
adhesion. Therefore the surface must be adequately prepared by
cleaning and slightly roughening to guarantee primer adhesion. This
is accomplished by using a conversion coating such as alodine. This
chemical process creates a ceramic layer over the aluminum that
coats the surface and provides tooth adhesion. Used aluminum must
have any primers, paints, or corrosion removed. Paint strippers are
used to remove old paint. After stripping old paint the corrosion
should be completely eliminated. Use fine sandpaper, Scotch Brite
pads, or aluminum wool. Never use steel wool or a steel brush. After
the corrosion is removed the old aluminum should be acid etched.
This is simply a process of washing the aluminum with a product
such as Poly Fiber's E-2310 Acid Etch diluted with water. An acid etch
removes oil and light corrosion while etching or roughening the
surface to provide a firm primer bond. The part is then thoroughly
rinsed. Next wash the surface with E-2300 Conversion Coating that
inhibits corrosion and further enhances primer adhesion. After this
step the part is rinsed and allowed to completely dry. Once again,
new aluminum surfaces need only be treated with a conversion
After the aluminum (new or old) has been properly cleaned and
treated, it is then primed. I would recommend using a two-part
epoxy primer. An epoxy primer will insure corrosion protection and
also provide a bonding surface for most topcoat paints. Very often,
polyurethane topcoats will lift or wrinkle primers other than epoxies
much as a paint stripper would do. A primer is necessary to provide a
bond between the metal and the final topcoat paint. The primer coat
should be applied according to the manufacturers directions. Usually,
two light coats will be applied. Heavy coats should be avoided.
Steel surfaces are much more susceptible to corrosion problems in
the form of rust. This rust must be completely removed prior to
priming the part. Of course, any old paints or primers will usually be
stripped. Certainly, if you are going to paint over the existing topcoat
you must still deal with any rust that might be present. Removal of
old paints can be accomplished with a bead blaster or sand blaster.
However, this must be done without pitting or damaging the metal.
Using the proper amount of pressure in blasting is essential. Once
the structure has been stripped and the rust eliminated, the metal
must be protected within 1-2 hours. Be sure to have the primer and
spray equipment ready before you begin blasting or cleaning. Rust
will begin to form on a bare steel surface within a very short period
of time. Just as with aluminum, after cleaning the structure prime it
using an epoxy primer. Be sure to clean the surface with a surface
cleaner just prior to priming.
Wooden surfaces are usually covered with fabric. They still must be
properly prepared to prevent rotting problems from moisture. Usually
the part will be dry sanded and then varnished using a two-part
epoxy varnish. Solvents used in fabric covering systems will "lift"
most varnishes other than epoxies. If you plan to paint directly over
the wood itself, an epoxy varnish must be used.
Fiberglass parts should be sanded smooth and primed using an
epoxy primer. Of course, if you are building a composite airplane the
surfaces must be filled and primed in the manner discussed in a
previous article on composite construction.
As a general review, all surfaces must be cleaned, any corrosion
removed, and then primed prior to painting. Epoxy primers come in a
variety of colors. The most popular colors are green, yellow, and
white. White colors are much easier to cover with final topcoats. It is
your choice. Zinc chromate has been used for years as a primer.
However, its popularity is decreasing with the advent of epoxies. Zinc
chromate should not be used if you plan to apply polyurethane paint.
The elapsed time between priming and applying the topcoat will vary
depending upon the brand of paint used. Different manufacturers
use varying times. Usually, an epoxy primer should completely cure
and harden prior to applying the topcoat. That process takes several
days depending upon the temperature and humidity. Often, a full
week is needed. The primer then needs to be scuff sanded to obtain
the needed adhesion for the topcoat.
Proper sanding is a very important step in acquiring a high quality
finish. If you want a high gloss you will spend time sanding. Sanding
is usually accomplished using wet/dry sandpaper and water. The grit
of sandpaper used is dependent upon the surface and its roughness.
Usually 180 grit all the way to 600 grit or higher sandpaper is used.
Sanding actually flattens the surface of the object you are painting. It
also removes any imperfections that may be present such as small
pieces of dirt. You should use a sanding block when possible.
Pneumatic orbital sanders can be used in larger areas. When holding
a piece of sandpaper in your hand, fold it in thirds to maximize the
use of the sandpaper. You can more efficiently use the paper this
way. A straight back and forth movement is preferred over a circular
movement. Sanding 45 degrees one direction and then 45 degrees in
the other direction works well.
You have a choice of the type of topcoat paint along with a large
number of brand names. Enamel is a paint that is commonly used on
aircraft surfaces. These paints are sprayed over epoxy primer after
being thinned to proper consistency using enamel thinners. A light,
mist coat is first sprayed on and allowed to dry for a few minutes
until it is tacky to the touch. This is then followed by a full coat of
enamel. One full coat may be sufficient or another may be sprayed if
desired. The use of enamel is not as popular as it was in the past.
A second type of topcoat paint is acrylic lacquer. This paint has a low
solid content that makes it more difficult to apply. Acrylic lacquer
should be thinned using the proper thinner and then a very light tack
coat applied. An additional 4-5 cross coats of paint will then have to
be applied allowing about 30 minutes drying time between coats. (By
the way, a cross-coat is defined as moving the spray gun north and
south followed by east and west: one cross coat).
Polyurethane paint is probably the most popular choice for a topcoat
today. It is very durable and provides a high gloss finish. It is also
chemically resistant. These paints have a high solid content and they
cure very slowly which means they continue to flow out for a long
period of time. This flowing out process forms a very flat surface that
gives the surface a high gloss look. Polyurethane enamels are mixed
with a catalyst prior to use. They are then reduced to proper
viscosity for spraying. A very light tack coat is first applied followed
by one or two full coats. One problem inherent in polyurethanes is
the thickness of the film applied. If the paint is applied too thick it
may tend to crack over a period of time. This is especially true when
applied over fabric. The fabric on an airplane will flex and move
during flight. This movement coupled with the thickness of
polyurethane paint can present a problem. Polyurethane paints
designed for fabric airplanes are manufactured and should be used
when painting over polyester fabrics.
The one major problem encountered when using polyurethane paints
is its toxicity. Breathing the spray mist from polyurethanes may
cause severe sickness or even death. With this in mind, you must use
a forced air breathing system such as the HobbyAir system. You
should also protect your skin and eyes.
If you are painting a fabric airplane, you can use a specially designed
polyurethane topcoat, butyrate dope, or Poly-Tone. Butyrate dope
and Poly-Fiber's Poly-Tone are both very easy to apply because they
are more viscous. Therefore, they are less likely to run when applied.
No matter which paint you choose a few fundamentals apply. First of
all, the paint needs to be properly mixed. That means shaking the
paint on a paint shaker within one week of application. After shaking
the paint it should then be thoroughly stirred just prior to use.
Secondly, the surfaces should be wiped down with a paint cleaning
solvent using a clean rag. Then a tack rag should be used to remove
any dust. Thirdly, the paint should be properly thinned by following
the manufacturers directions. A viscosity cup can be used for the
thinning procedure. These are small cups with a hole in the bottom.
The cup is filled with paint and then the viscosity is determined by
the amount of time, in seconds, required for the paint to flow
through the hole until it begins to drip. The next step is to strain the
paint through a mesh paint strainer. This should be done prior to
placing the paint into the spray cup. If you are using a polyurethane
paint, you should strain the paint prior to mixing the catalyst.
Mixing the catalyst in polyurethane paints should be done in
accordance with the directions from the manufacturer. Usually, you
should let the catalyst react with the base paint for at least 20
minutes prior to spraying. Once you have mixed the paint you will
have approximately 5 hours before the chemical crosslinking begins
and the mixture begins to thicken. With that in mind, only mix the
amount of paint you will need for the job. If you mixed too much
paint you can place it in a freezer (not with food) overnight, remove
it and allow it to reach room temperature before spraying. The cold
temperature delays the crosslinking process.
When applying the final color coats, always be sure the paint you are
using has the same batch number. Slight differences in color can
often be found in different batches of paint. One solution is to open
all of the cans of paint you will be using and mix them together in a
large container. They can then be poured back into the original
containers after being mixed.
Other chemicals in addition to reducers that you may encounter are
retarders and accelerators. A paint retarder very simply is a solvent
that slows the drying time of the paint. It is added in proportion to
the directions on the paint can. Retarders are generally used in high
temperature or high humidity conditions. Accelerators have the
opposite affect. They speed up the drying time. Accelerators may be
required to help the drying process in cool temperatures.
The final preparation of the aircraft involves protecting every
component part that will not receive paint. This means the
windshield, if installed, needs to be protected. pitot tubes, static
ports, tires, etc. must all be covered. Be sure to use a good quality
butcher paper to cover these items. Do not use newspaper, the print
will often come off onto your airplane's surface. The painting
equipment and the paint facility were discussed in detail last month.
A list of general tools and equipment is:
Drop cloths to protect the floor.
Fine-line masking tape
Butcher paper or masking paper
Orbital sander (optional)
Single edge razors
Soup ladle for dipping paint
Hand held agitator to use with pneumatic drill
Now let's discuss the actual process of painting. The most important
aspect of learning how to paint can be summarized in three words:
PRACTICE, PRACTICE, PRACTICE. Do not practice on your airplane.
Get several pieces of cardboard and learn how to properly set up the
spray gun. Then practice the spraying technique on the cardboard.
Next, spray pieces of metal lying flat on a surface. Then spray the
metal pieces hanging vertically. When you feel really confident, buy a
piece of stovepipe or a large diameter pvc pipe and paint it. Stand it
vertically and paint the entire piece. When you can do this without
major mistakes you are ready to begin on your airplane. If possible,
always begin the painting process with a small control surface.
gun to the
The actual adjustment of a spray gun depends upon the equipment
you are using. The manufacturer should provide you with a set of
instructions on setting up the gun. You should use the type of spray
gun nozzle recommended for the type of paint you will be spraying.
The gun should be properly adjusted each time you spray. A test
pattern should always be sprayed on a piece of cardboard before
beginning to paint. The normal pattern for a spray gun will be fan
shaped. To begin the actual application procedure, hold the spray
gun approximately 8 inches from the surface you will be painting.
Spreading your fingers as illustrated in Figure 1 will usually
approximate this distance. This distance may vary somewhat
depending upon whether you are using a HVLP system or a pressure
spray gun. The spray gun should be far enough away so the paint
does not run or sag when applied and close enough to lay on a wet
coat. To prevent the paint from being uneven, it is imperative that
the gun be held exactly perpendicular to the surface. If it is tilted the
paint will be heavier on one side and lighter on the other. See Figure
2. The spray gun should then be moved parallel to the surface only
the distance you can comfortably move your entire arm while
keeping the movement exactly parallel. See Figure 3. If the gun is
moved in an arc the material will be applied heavier in some places
and lighter in others. You should squeeze the trigger of the gun just
prior to beginning the paint stroke and release it just before it is
completed. You then should move up or down approximately fan
width and begin the next pass. You must overlap the passes to
achieve an even build-up. Each pass of the gun will usually apply the
paint more thick in the middle with a tapering off on each end.
Remember our definition of a cross-coat, one pass north and south
followed by a pass east and west.
If the gun is not
to the surface, one
side of the spray
pattern will be wet
with a tendency to
run, and the other
side will be rough
Proper lighting is absolutely essential when painting. For best results
try to position your lights 45 degrees to the surface being painted. By
looking into the glare of a light you will detect heavy and light areas
of material. Of course, heavy areas will often result in sags or runs.
Paint the edges of the structure first. Edges often do not receive an
adequate amount of paint. Painting them first will solve this problem.
If at all possible, paint on a flat surface. Of course, that is not always
feasible. Paint will sag or run much more easily on a vertical surface.
Spraying in corners and around corners presents a problem. Practice
in areas such as this to establish the proper technique. As a general
rule, spray the corner first whether it is inside or outside then you
can blend the paint in with subsequent strokes.
As I mentioned last month, it is much easier to paint your airplane
prior to assembling it. Practically speaking many builders will
assemble their airplane, test fly it, and then paint it. If you do paint
your airplane while it is in one piece there is a definite sequence you
should follow. First of all, you will want to paint ends and leading
edges of surfaces. Paint the bottom of the airplane first by beginning
at the tail. Spray from the tail control surfaces all the way up the
fuselage to the engine then spray the underneath side of the wings.
It is much easier if you can persuade another painter to help you.
That person can paint at the same time you are painting with one of
you staying slightly ahead of the other. Often it is impossible for one
person to reach entirely across a wing. The trick of the entire process
is to keep the surface wet all the time.
After spraying the underneath side of the airplane you then should
spray the vertical stabilizer, the top of the tail surfaces, the top of the
fuselage, and then the top of the wings. It is more difficult to paint
the airplane when it is assembled. Overspray is the problem. You
must keep overspray off the surfaces you have finished. I would
recommend visiting a local paint shop and watching their techniques.
There are a number of ways to do this and each painter has a trick
or technique. When you paint the airplane unassembled the
problems are minimized.
As a rule of thumb, a white coat of paint should be applied prior to
final colors. This will provide better coverage with less material and
also bring out a more brilliant color in the final coats. White primer
will serve this purpose. Do not try to cover red paint with a lighter
color. Red should always be the last coat sprayed.
The paint spray gun
must be moved
parallel to the
surface. If the gun
is moved in an arc,
the spray pattern
will be thick in some
spots and thin in
Once you have completed the base color you will then be ready for
the trim colors. A sufficient amount of drying time should be allowed
once again according to the manufacturer's recommendation. Usually
at least 12 hours is needed. Some poly urethane manufacturers
direct users to wait at least one week until the paint is more fully
cured. After one week the area to be painted should be lightly
sanded to provide tooth adhesion. Use the best grade of masking
tape available. I would recommend 3M's fine line tape that is
designed for trim use. Remember, do not use newspaper to cover
areas only good Kraft paper. Apply the tapes and then use a small
plastic squeegee to press down the edges of the tapes just prior to
spraying. Pull the tapes off when the paint dries to the touch, usually
1-2 hours. When pulling the tapes off pull toward the new paint. Do
not leave the tape on for a long period of time.
You will encounter problems. Runs and sags are perhaps the most
common. These usually form as the result of improper spraying
techniques, such as the gun too close to the surface, or the material
is too thin. When you create a run or sags simply stop and let the
paint dry. If it is a polyurethane you should let it dry several days.
Then go back and sand out the run or sag and respray the area. An
orange peel look is also a common problem. That usually results
from the air pressure being too high in a pressure gun, the paint too
viscous, or improper solvent. Blistering of the paint is a result of the
surface not being properly primed or moisture being present on the
surface. A coarse finish is another problem that occurs because the
surface was not thoroughly cleaned.
The final step is the one everyone dislikes: clean-up. If you are going
to preserve your spray gun you must clean it thoroughly. This means
taking the gun apart and cleaning it with a solvent, either reducer or
MEK. Remember, protect your hands from any solvent. After a
complete cleaning some painters will actually leave the nozzle of the
gun in a solvent until the next job. You also want to properly dispose
of any waste.
Why are you waiting? It is time to buy that paint outfit and begin. I
believe with proper equipment, a modest facility, and a lot of practice
you can apply a high quality finish on your airplane.