Exhausting Overspray in the Home Shop
Spray guns, especially the high-volume, low-pressure (HVLP) type with turbine-supplied air, have become fairly popular with amateur woodworkers. Like all spray systems, turbine HVLP guns transfer the finish from the can to the wood faster than brushing and produce a more level surface (no brush marks).
Also, like all spray systems, turbine HVLP guns create overspray, though considerably less than high-pressure guns. This overspray should be exhausted to remove explosive vapors, for health reasons and to keep the dried particles of finish from settling back on the finished work and other objects in your shop. Rarely is this need for exhaust, or ways of accomplishing it, mentioned in ads for spray equipment or in articles about HVLP spray guns or spraying.
Commercial Spray Booths
Professionally equipped shops and factories use commercially made spray booths to exhaust overspray. Essentially, a spray booth is a box that’s open at one end with an exhaust fan at the other and filters in between to catch overspray (see the drawing on the next page). Commercial spray booths have the following features:
• Steel construction for fire safety.
• Filters to catch and hold overspray before it gets to the fan.
• A chamber for collecting the air to be exhausted. This exhaust chamber makes it possible for air to be drawn uniformly through a much larger square footage of filters than the simple diameter of the fan.
• A large enough fan to create an air flow of 100 feet-or-more per minute, which is enough to pull “bounce-back” overspray away from the object being sprayed. The fan and motor are also “explosion proof” to eliminate sparks that might cause a fire or explosion from contact with solvent vapors. (Beware that a buildup of vapors can be ignited by a pilot light in your furnace, your water heater or from another source at home, too.)
• Side walls and a ceiling to create a work chamber or “tunnel” for directing the flow of air through the filters.
• Ceiling and sometimes side lighting so the operator can see a reflection off the surface he or she is spraying. (Working with a reflected light source is the only way an operator can know if the finish is being applied wet and without orange peel, runs, sags or other problems.)
Commercial spray booths are an essential tool for production shops, but these booths are too large, too expensive ($3,000 to $5,000 minimum) and require too much make-up air (heated air to replace the air being exhausted) for almost all home shops. If you are using a spray gun on an infrequent basis at home and have to work inside to avoid cold, wind, bugs, falling leaves and so on, you should consider building your own modified spray booth.