Aircraft Building: Painting

The cost of a professional aircraft paint job varies from ... paint, what paint to use, the paint- ing process itself, and ... percent) ends up as over spray, which drifts ...
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PAINTING is A CHALLENGE THAT most builders and restorers w i l l face at some point, and it sometimes strikes fear in their hearts. Why? Even though builders and restorers can weld the perfect joint, create flawless composite layups, and rivet sheet metal to perfection, how well they paint, and the colors and scheme they choose, determines how people will judge their aircraft. Striving for the perfect paint job is utmost in the minds of most builders and restorers. Because you want a perfect paint job, the immediate question is, "Should you do it yourself or hire a professional painter?" The answer—it depends. If you invest in the proper painting equipment, create a place to paint, and spend the time practicing, you can achieve the perfect finish on your airplane. The cost of a professional aircraft paint job varies from one location to another, but it's not unusual to pay from $3,000 to $10,000. Professional aviation paint shops have the proper facilities, top-of-the-line equipment, 98

DECEMBER 2000

Painting How to do it yourself RON ALEXANDER

You can give your airplane a professional paint job even if you have no experience because today's paints and application equipment make the process much easier.

and experience in aircraft preparation as well as painting. Leaving your airplane with a painting pro does not affect the FAA's requirement t h a t homebuilders construct the major portion of their aircraft. But you can give your airplane a professional paint job even if you have no experience because today's paints and application equipment make the process much easier. Pros will paint an airplane in less time and with less effort than the average homebuilder, but it's your airplane! You've built it from the beginning and paid close attention to the smallest detail, and you'll likely dedicate the same attention to the final finish. Although most homebuilders won't have the pro's bug- and dirtfree work environment and experience, all homebuilders have two major advantages: perseverance and the desire for perfection. With these two characteristics you are capable of a professional-looking paint job. To help you achieve a perfect finish, this is the first of a series of arti-

tube, creating a vacuum that feeds paint to the nozzle.) Get a good quality gun and ensure that the cup's vent, which allows air to enter and replace the paint you spray, lets you position the gun horizontally without dripping paint on your surface. For larger jobs, such as applying dope to fabric-covered airplanes, you can use a "pressure pot," w h i c h Nozzles and needle parts control the volume holds 5 gallons, but they and width of the spray pattern for specific Painting Equipment are hard to clean and their types of coating. Good painting equipment is es- supply hoses are subject to sential to the success of any painting c o n t a m i n a t i o n and must be reproject, and like any job, if you placed regularly. don't have the right tools, you might Spray gun nozzles are as imporas well not start work. Avoid using tant as the gun itself, and different airless spray guns, Power Painters, or types of coatings require specific anything designed to paint houses. nozzles. The coating manufacturer Invest in the right equipment. Yes, will recommend the nozzle to use it'll cost you more, but it will last a with its different products. To avoid lifetime. You have a number of dif- contaminating the coating, don't ferent spray systems to choose from. forget moisture traps and filters for Suction Gun Powered by your air compressor. Compressed Air: This system has A disadvantage of high-pressure been in use since the 1930s, and it painting systems is that the atomstill works great. You need an air ized paint reaches the surface to be compressor with an ample storage painted at a high velocity. The result tank capable of delivering 50 to 60 is that many of the paint particles This conversion gun converts highpressure compressed air for use in psi to the spray gun. A small 2- to 5- bounce off the surface and gather in high-volume, low-pressure applicahp compressor should be adequate. the air in the form of a cloud of tions. Often available for less than $500, a "over spray." compressor should be in every shop More specifically, the spray gun regardless of your painting plans. mixes air and paint to create a fan- use lower air pressure (3 to 5 psi) To measure the pressure when shaped spray p a t t e r n , and the to atomize the paint. In addition, you pull the trigger, you should high-pressure air creates "wingtip low pressure pressurizes the cup have a pressure gauge at the gun, vortices" (over spray) at the end of and forces the paint to the nozzle. and it should read 50 to 60 psi the spray f a n . Often a large per- Because the velocity of the atomwhen you pull the trigger. To main- centage of the paint used (30 to 40 ized paint is much lower, most of tain this pressure while painting, p e r c e n t ) ends up as over spray, it adheres to the surface instead of the compressor must supply at least which drifts around the paint bouncing off and ending up as 90 psi at its outlet and have ade- room until it settles on the floor or over spray. To achieve t h i s performance, quate storage. is caught by the ventilation sysFor smaller jobs a 1-quart cup tem's filter. At the price of paint, HVLP spray guns are designed to supplies paint to the gun. The high- that can amount to a lot of money gather the air inside the gun and send it downstream with as little pressure air sucks the paint from the passing through a filter. cup and atomizes it, allowing you to High-Volume, Low-Pressure pressure drop as possible. To reduce apply the resulting "mist" smoothly Systems: Newer paint application internal turbulence that can impede on the part. (When you pull the technology, the high-volume, low- this airflow, HVLP guns must be matrigger, the high-pressure air blows pressure (HVLP) system, reduces chined to closer tolerances than over the top of a small paint supply over-spray waste. These systems high-pressure spray guns because cles that will prepare you to paint your airplane or, at the very least, prepare you to paint various parts of your airplane. We'll look at the equipment you'll need, where to paint, what paint to use, the painting process itself, and a number of other subjects related to painting. Even if you decide to hire a pro, this series will help you understand what the professional painter is doing (and it may help you select the right shop to do the job).

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they cannot use higher air pressure to overcome flow problems, like you can in a high-pressure system.

compressor, and because H V L P systems run on 110 volts, you can plug them in anywhere. They also solve the moisture problem. One of the most significant problems a painter faces is the presence

There are two types of HVLP of moisture that can splatter onto a sprayers, a turbine system and a surface along with the paint. The conversion spray gun. We'll discuss turbine's heat removes up to 95 the turbine first. Turbine HVLP sys- percent of the moisture in the air it tems don't use an air compressor. In- provides to the system. stead they have a turbine not unlike Perhaps most important, HVLP that in a vacuum cleaner. (Several systems enable beginning painters years ago, Electrolux offered a to achieve a professional finish, and sprayer attachment with its vacuum they are easier to use than convencleaner, the first HVLP system.) For tional spray outfits. Painters with exthe HVLP gun to work effectively, perience using conventional spray the turbine need only supply a large outfits will have to learn the differvolume of air—not high pressure— ent techniques an HVLP system reabout 50 cubic feet per minute at quires because it expends heavier amounts of material than convenless than IO psi. A high-speed centrifugal blower, a tional guns. Remote paint pots that 110-volt motor, turns the HVLP tur- hold 2.5 gallons are also available. bine at near 20,000 rpm, and the reHVLP units, complete with a t u r b i n e , 30 feet of hose, and a sulting friction heats the air, which eliminates the moisture from the air spray gun, cost between $700 and that atomizes the paint. This heated $800, and the gun represents the air also reduces the flash or drying majority of the purchase price. You time, which helps minimi/.e blushing. w i l l also w a n t to purchase addiThere are two types of HVLP spray tional spray gun nozzles for the guns: non-bleeder and bleeder. type of paint you are spraying. ReWhen you release the trigger of a mote 2.5-gallon paint pots are also non-bleeder the airflow stops. The an option. To reduce the temperaair flows continuously on a bleeder t u r e of the air reaching the gun, HVLP gun, and the trigger only con- many people recommend an additrols the flow of paint. A bleeder gun tional length of hose. HVLP Gun Powered by Comgives a better finish, but it requires more care because the continuous pressed Air: This "conversion airflow can blow dirt and other de- g u n " is the choice of most a u t o body shops. It converts high-presbris onto the painted surface. HVLP systems offer many advan- sure compressed air for use in a tages to other systems. The reduced high-volume, low-pressure gun that over spray cuts wasted paint by 30 has all the advantages of an HVLP to 40 percent. You don't need an air system without the need for the

mt-propeller turbine unit. Most of these units come with gravity-fed cups that are mounted on top of the gun. Gravity-fed guns have good balance and give you a better view of the surface you're spraying. Most conversion guns are expensive, costing between $300 and $500, and they require a highcapacity air compressor with moisture traps. Other Types of Spray Equipment: There are two other types of spray equipment—airless and electrostatic—and they are used primarily in production work, not in the home shop. Airless sprayers force paint through a small tip opening at extremely high fluid pressures (usually 1200 to 3600 psi), which enables them to spray almost any coating. They are rarely used by the aircraft industry. Klectrostatic spray systems charge the paint at the tip, and because the paint's polarity is different from the surface being painted, the surface's polarity attracts the p a i n t p a r t i c l e s and wraps them around the object.

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Paint Settings

Most spray units will handle different types of paints, but to get the desired result you must set the correct combination of needle, nozzle, and "air cap" (also called a "fluid set"). Thick paint uses one setting, and t h i n paint uses another. The unit's manufacturer will recommend the correct settings. There are no universal sizes, so refer to the manufacturer's literature and follow the recommendations even though it will require purchasing several different sets. To get a good finish, spray equipment is mandatory, and keeping the u n i t clean is e q u a l l y i m p o r t a n t . Avoid rented equipment that is usually left dirty and often has been abused. Spend the money necessary to get what you need. You will be money ahead in the long run. Next month we'll discuss paint booths, lighting, and safety.

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