An "air handling" system can be a typical large squirrel cage fan found in many houses, offices, or apartment houses. There seems to be enough of these systems floating around used to fill the needs of the spray room builder, but if you must buy one new get at least a ten ton unit. My room is 20' wide, 30' deep, and 13' high. With this ten ton unit, shown hanging from the ceiling outside the spray room, my air changes about once every 3 to 5 minutes depending on the size of the vehicle being sprayed. The best thing about this all electric system is that I can also control the temperature in the room using a thermostat in the room. With the use of baffles I can bring air in to the booth from outside the building, from other parts of the shop, or recycle the air in the room in order to warm it more thoroughly.
I duct my air into these 4' x 6' homemade filter boxes. The filters cost me about $80 each to replace every two years of pretty steady use. Those panels are hinged 2" x 3" frames with 1/4" wire mesh under the filters. I use a product from American Air Filter (800-501-3146) called "Fusion Media." You buy it in precut sheets, you dictate the size.
Pressurizing the room is done by the incoming air while air in the room is forced out. Using this concept of a "positive pressure" environment, you overcome many problems created by the "negative pressure" environment created by the fan in the window or wall blowing out. Dust control is the main reason for this pressurizing. A negative pressure brings dust from every crack and crevasse in the room while positive pressure means you only need to deal with the incoming air through the filters.
I constructed the walls of galvanized steel studs and gypsum board, framing the openings with pressure treated lumber. The lumber that came in contact with the floor, walls, and ceiling was caulked well during installation so leakage is nearly non-existent. This wall went up in a day.
The doors are made of Styrofoam and set into the opening with the edge sealed with a plastic "V" strip on the sides and to and a rubber bead fastened to the bottom. It's held in place with a cross bar. When removed, it hangs on the wall out of the way. My booth has a normal size 3' x 7' door and a 4' x 8' door, so when the booth is not being used for painting it's easy to move between bays. Also note in the picture to the left, the light recessed in the wall and covered with plastic. Access to the light is gained from outside the booth.
Lighting is critical, if you can't see the surface properly that you're painting, your results will show it. Lighting does two basic things: first, it shows you variations in color; second it shows you variations in texture. The light needs to be bright enough to highlight these variables without being so bright that it overpowers your sight while looking at the surface from different angles. I use inexpensive 4 foot shop lights purchased from a chain hardware store. They're installed about 18 inches above the floor behind a sealed plastic cover. These two bulb fixtures are mounted on a hinged plywood panel which allows for bulb changing. I have two of these 4 bulb units on each side of the work space which allow for good color perception, and (because of the tube length) they also give a reflection which shows texture on the surface, allowing me to tweak the surface for orange peel etc.
The floor is pitched slightly toward the overhead door which allows the floor to be washed and rinsed easily. The floor is painted in order to keep the concrete sealed so that it doesn't generate dust. I was using an epoxy floor paint for a long time then found that I liked latex floor paint much better, it's easier and cheaper to use, and I do it more often to keep the place bright and clean. I fooled with many colors on the floor but have found that a light gray worked best for me, it didn't distort paint colors and it helped reflect a little light that darker colors would absorb. Before any full paint job or other critical spraying operations the floor is dampened to help with dust.
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