I was visiting my old friend, Kevin Corrigan. We had played high school ball together and hadn't seen each other for a few years. "Kevin, you are always 'poor-mouthing' me about how you can't make money painting aircraft. This is nonsense. What are you doing in your operation that is costing so much money?" Kevin and I were discussing the painting of singles and light twins in his office. Outside in the hangar was a beautifully refinished Extra EA-400 racing plane that had a finish you could see your reflection in. It was just gorgeous. The skill and professionalism that was incorporated into that finish was something that was amazing to see.
"I paint the planes and get the colors and finish just right, but I have a lot of re-work. It takes a lot of time to buff it correctly and get the imperfections out of the surface. If you get just one little dust particle in the surface, you can see it from a long way off and it just has to be fixed," said Kevin, fidgeting with his deskpad and pencils.
"Maybe we should look at your operation," I said and we went into his hangar. It was typical of most airplane paint hangars in this industry, having a small exhaust filter chamber along the rear wall. Air was drawn in through the hangar door opening and was unheated. The building was a pole building with metal siding and metal ceiling rafters and roof purlins. The booth was lighted by eight metal halide pendant fixtures and was very dimly lit.
"Kevin, we have to talk," I said as I made a few notes. "I see several areas of improvement that you should make. The first is to reduce the risk of fire by changing your lights to explosion-proof lights, which is going to be costly, but will probably save you the loss of an aircraft by fire. The second is to ventilate better. The exhaust filter and fan are good, but they are way undersized. I am afraid you will build up vapor concentrations that may be within the flammable limits. With more exhaust, you will reduce that risk."
Looking further, I said, "The air replacement is natural draft and it comes into the building through cracks and door openings and brings with it all the outdoor dust and pollen and such things as bugs and birds. None of these help you reduce re-work. The best way to be sure your painting environment is clean is to install an air replacement unit that will bring in filtered and heated air directly into the painting space."
"That's fine, but how much air do I need to exhaust to be safe from explosion?" he wondered.
That is a good question, of course, and much study has been put into the special needs of aircraft booths. There are actually two considerations. The first is to reduce explosive concentration to prevent risk of fire. NFPA-33 says to keep the concentrations below 25 percent of the Lower Flammable Limit (LFL). The second is to have enough airflow to remove overspray and filter out contaminants. The second always requires more air than the first.
Let's look at standard paint booths. These facilities are designed for objects that fill the booth space. If you look at trucks, buses, construction machinery, and the like, you see that the booth is just 3 to 4 feet wider and higher than the object to be painted. This extra space allows a painter free range of movement. The object itself sitting inside a standard heavy equipment paint booth fills about 80 percent of the available booth cross-section, whether downdraft or crossdraft (see photo at right).
In contrast, an airplane, when viewed nose-on, occupies about 15 to 20 percent of the booth cross-sectional area if the booth is designed in a rectangular layout, which is typical. This is an immediate problem with booth airflow, since the acceleration of air that you get as air moves around a bus, for instance, aids in the removal of overspray. Because of the relatively slow movement of air, overspray hanging in the air falls on the plane surfaces and makes an imperfection in the surface. The best way to reduce buff and rework time is to control the overspray. Airflow is the only way to control overspray, but you need to remove more air to control overspray than you would to just control risk of fire.
A look at crossdraft and downdraft paint booths By Rich Thelen From top: Crossdraft with forced draft plenum doors. Downdraft showing filter house. Fighter booth. It goes...
By Emily Refermat Whether covered from head to foot in personal protective equipment painting a Cessna Citation in a cross-draft paint booth or wearing gloves and a respirator spraying a wheel...