Exhausting Overspray in the Home Shop

Making Your Own
With a note of caution that doing any type of spraying in your house, with or without a spray booth, could affect your homeowner’s insurance, here’s how to build a safe, inexpensive spray booth that will be adequate in the volume of air and overspray exhausted and take up very little space.

The spray booth consists of a fan with a separate motor connected by a fan belt, one or more furnace filters and plastic curtains.

Your choice of fan is determined by the amount of air, measured in cubic feet per minute (cfm), you want to move and is a trade off between better exhaust of overspray and reducing the need to supply heated make-up air on cold days. In other words, the more air your fan moves, the better the exhaust but the more windows you’ll need to open at the opposite end of your shop and the faster the heat in your shop will be lost. Generally, the larger the fan and the more sharply angled its blades the more air it is capable of moving.

To mount the fan, construct a box from plywood or particleboard approximately one-foot deep with both ends open. The dimensions of the four sides should be adequate to hold the fan at one end and furnace filters, which should be efficient enough to trap all overspray particles before they reach the fan, at the other.

Cut a slot on the top of the box large enough for the fan belt to pass through and mount a motor adequate in horsepower to drive the fan on the outside of the box. A 1/4- to 1/2-horsepower motor (1,725 rpm) would be typical. If you are going to spray solvent-based finishes, an explosion-proof motor is best. A standard TEFC (totally enclosed, fan-cooled) motor will do if you are going to spray only water-base finishes. Either way, the motor needs to be enclosed in a box to keep overspray from building up on it.

Place the box with the enclosed fan in a window, possibly resting on a stand just in front of the window and seal the spaces between the box and window opening. Then hang plastic curtains from the ceiling on either side of the fan running out about 8 feet from the window wall. If the window is near a side wall, you could use it as one side of your booth instead of a curtain. You want the curtains to be wide enough apart so you can stand inside, or just outside, the tunnel when spraying.

The best curtains to use are heavy, fire-resistant, “Industrial Curtain Partitions” with supplied ceiling tracking that are available from auto-body supply stores, Grainger’s or Goff’s Curtain Walls (800-234-0337). But you can use any type of plastic sheeting with the downside being that if the plastic is very light, it might be sucked in a little by the exhaust fan.

Mount the curtains to tracking on the ceiling so they can be pushed back when you aren’t spraying and pulled open when you are. This way, you lose almost no space in your workshop.

Don’t forget that you still need to wear a respirator while spraying with this exhaust system. I recommend a respirator with organic-vapor cartridges. (Lee Valley Tools sells one for $37.50. Phone: 800-871-8158).

For lighting, recess a four-tube, four-foot fluorescent fixture between the joists in the ceiling as close as possible to the window and shield the lights from contact with overspray by inserting glass plates between the light and the ceiling. For the best color balance, use full-spectrum fluorescent bulbs.

To avoid a fire hazard with your spray booth, it’s essential that you keep it clean. Sweep the floor after each job and clean or replace the filter. If finish starts to cake on the curtains or fan box, clean or replace them. PW

Sources
Probably the biggest decision you have to make when building this system for your home is in selecting a fan. You have a few choices here.

All-in-one Units: By spending a little money, you can make things simple. Grainger (www.grainger.com; 888-361-8649) sells exhaust fans for areas with flammable or explosive vapors. Some examples include:

• Dayton 16″ Panel Fan; cast aluminum blades; max cfm: 1,215; stock # 3XK37, price $435.25.

• Dayton 16″ Exhaust Fan; cast aluminum blades; max cfm: 1,980; stock #4C369; price $457.25.

• Other sizes are available.

Motor-Separate Units: You also might be able to find a belt-driven fan without a motor from an industrial supply company. With these, you can mount a TEFC or explosion-proof motor outside of the path of the airflow as described in the article.

FILTERS: Use the standard blue woven furnace filters you can find at any home center store. Make sure that they are not too dense; some filters can greatly reduce your airflow. Be sure to clean the filter after each project.

Bob Flexner is a nationally known finishing expert in Norman, Oklahoma, and the author of “Understanding Wood Finishing.”