Aircraft are generally painted for two reasons-appearance and protection. An otherwise plain airplane can be made much more appealing with a creative paint design and color scheme. The prep process, primer coats, and finish paint offer excellent protection from the aircraft fluids, airborne dust and dirt, bugs, bird droppings, and the elements.
The Rules
There are no regulatory requirements for the technical aspects of the paint job itself. Generally accepted industry practices and procedures for quality and application apply here. A paint job is considered a maintenance procedure and as such is covered by Part 43 of the federal aviation regulations (FARs). The paint job itself is considered a minor repair and must be signed off in the aircraft maintenance records under the provisions of FAR 43.9.
Some aircraft require rebalancing of the flight controls following a paint job and some do not. The aircraft maintenance manual will specify this. If the flight controls must be rebalanced, this is a major repair and requires the appropriate signoff and paperwork. If the aircraft has been painted previously and is now being repainted, the weight-and-balance change is generally considered negligible. If the aircraft was not previously painted, a weight-and-balance measurement should be performed and the appropriate maintenance record entry made.
When an aircraft is repainted, the design and placement of the registration numbers must be correct. FAR Part 45.21 through 45.29 covers this area. Generally speaking, the N number must be 12 inches high, eight inches wide, have no ornamentation, contrast in color with the background, and be legible.
You may come across an aircraft displaying a two-inch N number. If the two-inch number is the original factory-applied number, it is legal on aircraft displaying such numbers prior to November 2, 1981, and on aircraft originally displaying two-inch numbers that were manufactured after November 1, 1981 but before January 1, 1983. If these aircraft are repainted or the number is repainted, the two-inch number must be changed to a 12-inch number at that time. I have caught several repainted aircraft on this during annual inspections.
Preparing To Paint
The paint process begins with the preparation of the surface to be painted. As with many things, the quality of the finished product depends to a very great extent on the prep work done at the beginning. If the aircraft has been previously painted, the old finish will have to be completely removed down to bare metal for the best possible paint job. There are primarily two methods of doing this-chemical stripping and media blasting.
Chemical stripping is the traditional method of paint removal, but it is becoming increasingly costly because of Environmental Protection Agency requirements on facilities and by-product disposal and neutralization. Media blasting is increasingly being used to avoid these problems. Media blasting, which should not be confused with sand blasting, is most commonly done using very small plastic beads at low pressures and high volumes to avoid deforming the relatively thin sheet metal skin of the aircraft. Some systems use equipment that first blasts the surface and then recovers the media and paint particles. The media and paint particles are separated, and the media is reused while the paint residue is collected for proper disposal.
Media blasting has two added advantages over chemical stripping. The blasting process roughens the surface to create a better bond between the surface and the primer, and some amount of surface corrosion is removed during the blasting process.
Finishing Touches
There are several finishing products in general use on aircraft. Synthetic enamel is the traditional finish for metal airplanes and has been around for many years. Enamel is applied over zinc chromate primer and cures by the process of oxidation. Enamel produces a reasonably good gloss and does not require rubbing, but it is less resistant to chemicals and fluids than we might like, nor is it highly resistant to physical abrasions. Enamel dries slowly and must be applied in a dust-free environment.
Acrylic lacquer is the finish of choice for mass-produced aircraft because it dries quickly. Lacquer is applied over a two-part wash primer and can be applied as soon as the primer is dry. Lacquer is easy to apply because it has a relatively low solid content, produces a good gloss (especially if rubbed), and is resistant to chemicals, fluids, and weather.
The current favorite aircraft finish is polyurethane, which is a two-part chemically cured finish that is most often applied over a two-part epoxy primer. Polyurethane has a fairly high solids content and produces a very high gloss "wet look" finish, excellent chemical resistance, and the greatest durability of any commonly available finish. Polyurethane is definitely the finish of choice for high-speed aircraft, agricultural aircraft, float planes, and seaplanes. Polyurethane dries more slowly than lacquer but faster than enamel. It will dry to the touch in about five hours, but will continue to flow and level for several days. Polyurethane is a costly finish to apply. The finish itself can cost close to $100 a gallon for light colors and runs well over that for red shades. The chemicals in the product require serious personal protection measures for the applicators, including impermeable clothing and a continuous supply of fresh air for breathing.
Of course, not all aircraft are made of metal. Fabric-covered aircraft are most often painted as well. Since fabric is not as solid as metal and flexes in the wind, the paint must have a plasticizer added to provide long-term flexibility and prevent cracking. Fabric aircraft can be painted with enamel, but polyurethane finishes are far superior.
There are also several types of specialty finishes used in various places around the aircraft. Instrument panels are often finished with either flat black lacquer or wrinkle finish to prevent reflections and enhance instrument visibility. Wing roots and other aircraft structures used to enter and exit the aircraft are finished with a thick wing-walk compound that contains sand. This finish prevents slipping and protects the aircraft surface underneath from scratches and abrasions. Battery boxes and surrounding structures are finished with a special acid-proof paint that resembles tar to prevent corrosion from battery fumes and acids. The bottom hulls of seaplanes and the bottom surfaces of floats are finished with a special compound that is very similar to acid-proof paint. The finish resists abrasions from high-speed takeoffs and landings on the water as well as some amount of contact with sandy and muddy (but not rocky) bottoms. Engine and engine compartment parts are often finished with high-temperature paints to both enhance appearance and to provide corrosion protection. Properly selected and applied high-temperature paint can actually enhance the transfer of heat away from hot engine and exhaust parts.
Application Systems
There are two paint application systems in common use. The traditional system uses a spray gun powered by an air compressor that delivers high-pressure, low-volume air. The paint is atomized and mixed with air for delivery to the surface being painted. The paint must be thinned for spraying, but the thinner evaporates during the drying or curing process. This system works well but has several drawbacks, mainly overspray, which occurs around the edges of the main spray pattern. If this area is not properly masked off, the result is a rough, grainy finish that can be difficult to remove. Overspray also puts paint mist into the air that is hazardous to personnel and wastes costly finish materials.
A newer system called HVLP (high volume, low pressure) uses a different paint gun design and an air supply that is low pressure and high volume. The HVLP system has virtually no overspray and uses about 30 percent less finish to accomplish the same job. I have applied a red stripe on a white fuselage by masking only a foot on each side of the stripe. The result was perfect. The time savings in masking and paint will pay for an HVLP system very quickly.
Paint Maintenance
Once your pride and joy is back in service, there are several things you can do to maintain and prolong the life of the paint. Keeping the surface clean is a great start. Dirt and acids from the atmosphere will eventually wear on the finish. Bug residue can also quickly damage enamel and lacquer finishes. Waxing the aircraft is an excellent maintenance procedure for enamels and lacquers. It is of dubious value on a polyurethane finish but if it makes you happy to polish your airplane, have at it.
Related Articles
