How the box is built, where the moving air comes from and where it goes on the way out of the box are the basic issues about any booth. To achieve cleaner paint work, however, you must fully understand these basic issues, along with the bells and whistles - such as lights and doors - that make the booth easier to use.

The Building Blocks

Class I Division I locations where flammable solvents are being sprayed generally require walls and ceilings with a two-hour fire rating. The rating describes the ability of the enclosure to contain flames, hopefully long enough to extinguish them. Two-hour ratings are available by using metal panels, cement blocks or even double-layer, 5/8-inch sheet rock on metal 2x4s.

• Cement blocks - One problem with cement-block construction is the porosity of the blocks: OSHA regulations may call for "smooth" interior walls. Block walls would then have to be filled and sealed to obtain a smooth finish. Sealing the blocks also makes for cleaner paint work, eliminating any place for dust to hide.
• Metal panels - Most all manufactured spraybooths use metal panels to construct the spraying box. Some booths use single-thickness metal panels, and others use double layers with insulation sandwiched in between.

For many years, rust was a huge problem for spraybooths, just as it was for automobiles. Many booths were assembled on top of a 4-inch cement curb or up on treated 4x4s. The goal was to keep the bottom edge of the booth off the floor, where it would stand in water and soon rust.

These days, galvanized metal is used for most spraybooth construction. Just like the dramatic effect it's had on eliminating rust on automobiles, galvanized metal has also eliminated rust-through on spraybooth panels.

The panels themselves are flanged to interlock flush on the inside of the box, and most booths require caulking of seams and joints when construction is finished. Tip: If you haven't recaulked your booth lately, this would be a good time. Don't use the expensive autobody seam sealer, though - instead, buy the inexpensive stuff at the lumber yard. The finish on the panels runs the gamut from unpainted galvanized to high-gloss baked enamel with stainless-steel hinges and fasteners.

• Insulated metal panels - These are more expensive to construct than single-thickness, 18-gauge, rolled-edge panels because twice the metal is required (inside and outside), which makes raw-material costs higher. On a positive note, double-sided panels make for a quieter-running booth, and all that insulation absorbs sound, too. Double-sided panels also travel better than single-thickness sheet metal. You know how hard it is to get a replacement quarter panel all the way from the manufacturer to the technician's stall without a footprint in it? It's even harder to ship sheet metal across an ocean without damage.

Proponents of double-wall construction also contend that more heat will be retained inside the booth during any force-dry cycle. The main heat loss, though, comes when the exhaust fan carries the heated air away at 10,000 cfm, not through the metal sides of the booth.

Exhausted

Where the air to be exhausted comes from is the next big variable. For most body shop spraybooths, the exhaust fan will move out about 10,000 cfm of air. For most shops, the replacement air is simply sucked from inside the rest of the building.

When the room doors are hard to open and slam shut behind you, it's a tip that you don't have enough replacement air available. Also, when the fan is starved for air, the dirt problem inside the booth gets much worse.

That fan wants 10,000 cfm every minute. If it can't pull the required air through the intake filters, it'll pull it through the cracks in the corners and around the door edges. And since the replacement air is coming in from the rest of the body shop, it's full of dirt and dust.

The fastest way to cleaner paint work is to install an air-replacement furnace on the booth. This device takes clean air from outside the building, filters it, heats it (if necessary) and delivers it right outside the spraybooth's intake filters. Since there's much less dirt in this air, the intake filters do a better job and last longer than when the replacement air was dragged across every dirty car in the shop on the way into the booth.

It's also advisable to bring in slightly more air than is being exhausted to establish a positive pressure inside the spraybooth. For example, if the exhaust fan moved 10,000 cfm of air each minute, the air replacement should bring in 10,001 cfm of replacement air; now the pressure is greater inside the booth than outside. If you stood in front of the booth-door seam with a handful of dust, the dust would be blown off your hand by the escaping air - not sucked through the crack into the booth.

An air-replacement furnace to fit most automotive spraybooths will cost about $10,000 to $15,000. An existing crossdraft booth also would need an intake plenum, basically a metal box to hold air. By installing a metal box in front of the regular intake filters and ducting it directly to the air replacement unit, you have a closed system. This makes for much cleaner paint work and also offers the opportunity to speed production by force drying the paint.

Forcing the paint to lose solvent and crosslink quicker is possible by raising the temperature in the booth to around 140 degrees. Most air-replacement units are capable of either a 90-degree or a 140-degree temperature rise; this means they can intake outside ambient air at 20 degrees and, in one minute, raise 10,000 cfm of air to either 110 degrees or 160 degrees, depending on how large the burners in the furnace are.

Force drying the work in your current crossdraft booth will produce more paint jobs by the end of the week, even without buying a new booth.

Getting Direction

The direction of the airflow within the spraybooth can make a huge difference in the quality of the paint work. In a standard crossdraft-airflow design, the incoming air is dragged the length of the vehicle before being exhausted. In a downdraft airflow, the incoming air is passed over a single section of the car before being immediately pulled out of the floor exhaust; any contaminants in the air are gone before the paint can be

polluted.

• Downdraft airflow - This airflow is the most desirable choice for fast-drying, clean paint work. The vehicle dries faster because the 10,000 cfm of rapidly moving (100-plus feet per minute) air pulls the solvent out of the paint film very quickly. Note: Add approximately 10 degrees to the booth temperature when choosing a solvent for use in a downdraft because of all the moving air.

Getting the air to flow smoothly from top to bottom requires an air-exhaust trench below floor level. This additional construction cost can add several thousand dollars to installation expenses. Some shops choose not to purchase a downdraft booth because they're in a rented building, but the cleaner paint work possible with a downdraft will recover the cost of the pit construction quickly. Not having to sand and buff a farm field out of every paint job saves a ton of time, and when the time comes to leave the rented building, a truckload of sand and a couple hundred dollars in cement work will fill and cover the exhaust pit in about an hour.

If you absolutely can't dig a pit, you can buy a plenum (box of air) to sit beneath the booth to carry out the exhausted air. In this case, you would also need ramps to drive the car up into the booth.

• Semidowndraft airflow - If you can't make either downdraft alternative work, consider a semidowndraft. This spray box is installed at floor level, but the intake air comes in from the rear third of the booth ceiling. The exhaust is pulled from the front edge of the booth near floor level, which sets up a diagonal airflow across the car. This drags less dirt by the vehicle than a crossdraft does but passes over more of the car than a downdraft.

In any case where the air is delivered from above the vehicle, special air-balancing intake filters are used. Make sure you use replacement intake filters that meet the booth manufacturer's specs. Not all air-balancing filters are the same, and using a more or less restrictive type will affect airflow dramatically. Frequent changes of both the intake and exhaust filters are necessary to keep the work clean.

Good, Clean Fun

Whatever the design of your booth, the dirt in your paint work still comes almost equally from the car, the painter and the booth itself. As many shops have discovered, just buying a heated, replacement-air downdraft booth will not automatically provide clean paint work. Successful painting requires the painter wear lint-free coveralls and a fresh head sock. Prepping the car for cleaner work means a soap and water wash with a drenching water rinse. Using treated masking paper and even double masking the car will keep the work cleaner, too. If you can't afford a new booth, try a disposable paper suit, a few extra minutes with the water hose and some high-quality masking paper. Cleaner paint work doesn't always require new equipment - sometimes all it takes is some good old common sense.

Mark Clark, owner of Clark Supply in Waterloo, Iowa, is a contributing editor to BodyShop Business.

Regarding Regulations

Where the exhaust stack leaves the building, what size it is and how high off the ground it climbs are often regulated by state or local ordinances. New spraybooth permits are quickly becoming the norm across the country, so begin any new booth project with a call to the authorities first.

If you can't exit the exhaust out the side of your building, you need to know that before booth installation begins. If you must use a particular gauge or diameter duct work, find out before the contractor begins work. In fact, write down who (the booth manufacturer, the jobber or the shop) will be responsible for what. Every shop owner I ever talked with liked his new booth when it was all said and done, but many had a trying time getting it purchased, delivered and installed. Plan ahead to avoid the same problems.