One accurate description of an automotive spraybooth is "fire-proof box." In fact, the original impetus for vehicle spraybooths — outside of manufacturing — was fire containment. It turned out that once the enclosure was tight enough to contain a blaze, it was also tight enough to prevent the shop filler dust and gravel from sticking to the car. Tightly woven intake filters that would screen out the dirt but still allow for the passage of plenty of air were a logical next step in spraybooth evolution. Overspray traps on the downstream airflow were necessary to keep the sticky overspray from clinging to cars, cats and dogs a block away. In addition, issues like lighting, insulation, air replacement, force cure and air direction all evolved to answer the need of actual spraybooth users.
As we enter the new millennium, automotive spraybooths have never been better.
A word about the fire issue. By definition, the local fire inspector is responsible for keeping his community safe from fires. All those petroleum solvents in automotive paints burn quite well, so the fire department has determined that a body shop is more likely to catch fire than any other automotive-service business. Believe me, they’re not trying to make your life miserable; they’re just trying to control a likely source of tragedy.
With that in mind, a worthwhile suggestion is to begin any spraybooth project, from a remodel to a replacement, with the local fire authorities. Many shop owners have sad tales of the delays caused when the fire people and the booth people weren’t on the same page. Chances are, the local fire, electrical, mechanical and building codes aren’t going to change at your request. If you make sure your project meets the local requirements before you begin, the implementation process will be much smoother.
More Than Just Four Walls
The galvanized sheet metal forming the spraybooth cabin comes in several gauges as well as single- and double-wall styles. Neither of them burn very well. Double-wall cabins have a layer of insulation between the metal sheets. Single-wall booths are cheaper to fabricate but aren’t as quiet. Heat lost through either type of cabin wall is negligible. The lost heat streaks into the exhaust fan’s pull and is whisked right out the stack.
One important booth design element is the precision with which the metal cabin is fabricated. The more carefully the filter holes are cut and the squarer the panels, the tighter the filters will seat. And the more rigid the finished structure, the less likely it will spring leaks at the seams when the exhaust fan vibrates and doors slam. Whether the panels are double-wall or single-wall with clever bends and bracing for strength, sturdy cabin construction is the first step in clean paint jobs.
Features like powder coating, vinyl sheeting and clever panel interlock further differentiate construction styles. Once your cabin won’t burn, find or add the features that make painting easier.
Seeing the Light
Better lighting could make any painter’s life easier. "You can’t have too much light in a spraybooth," says one booth manufacturer’s president. I agree.
Whether you’re choosing a new booth or updating a current one, more light will be a welcome addition. If you intend to add lights, remember the ones within 3 feet of any door in the booth must be Class 1 Division 2 vapor-tight boxes. All others need only be sealed behind heat-tempered glass and wired to local code.
You’ll also get more useful light from more expensive bulbs. The $2 discount store bulbs generally don’t produce as much light as $5 or $10 bulbs. Color-corrected bulbs attempt to duplicate sunlight, under which the most accurate color matches are possible. If you elect to install color-corrected bulbs, make sure the illumination from them is equal to the ordinary bulbs being replaced. (Personally, I think the space in the booth is too valuable to be matching colors in. Get the color tinted to match before the car is in the booth and you’ll get more painting done every day.)
Effective intake filters that seal perfectly around each edge are a must. Crossdraft booths can successfully use several different styles of incoming filters. They can be cardboard framed, spun, pleated, sticky, pocketed or some combination. The goal is to trap the dirt that’s pulled toward the exhaust fan before it can stick in the wet paint.
No matter which style of door filter you use, the moving air would rather slip around the filter than pass through it. If air can find a crack, it will pass through the crack since it seeks the path of least resistance. So any time the filter doesn’t seat perfectly in its hole, air will rush through the gap instead of the restrictive filter. (The same is true for your charcoal respirator where it fits your face!)
Changing intake filters is a necessary but unloved job. If no one in your shop wants to do it right, consider hiring someone to make regular changes for you. Frequent replacement of filters will save money in heat, and cut and polish labor time. The finish will dry faster, too. Take the extra time to make sure each filter is seated perfectly in its opening. If modifying the mounting frame, rubber weather-stripping the edges or duct taping them in place will keep the air passing through — not around — the filter, it’s time well-spent.
The overhead filters in a downdraft booth are critical to making the whole thing work. Buying only the manufacturer-recommended filters can save lots of grief.
To better understand the role of overhead filters, let’s review what actually makes downdraft airflow possible. A plenum is a box of air. The overhead plenum on a downdraft booth fills with air forced into the box by the fan. The magic happens when the air won’t pass out of the box until the air pressure all across the filter face is equal. Called air-balancing filters, they’re a sophisticated weave that makes it possible to discharge air uniformly over the whole plenum surface.
Good ceiling filters are expensive. In many body shops, the desire to save money by purchasing cheaper filters results in poor booth performance. The tightly, carefully woven filters that cost $500 per set work much better than the $100 set sold by the guy in the roving "filter service" truck. Cheaper filters won’t balance the airflow and will result in a booth with lots of air movement near the center of the plenum but little at either end. Spend the money for the good stuff. Labor time is always the most expensive thing in a shop. Buffing every job to get it clean burns up far more money than new filters will ever cost. At a door rate of $36 per hour, you could buy new $500 filters every seven working days and be money ahead if you currently spent only two hours a day polishing out dirt.
Air Replacement Furnaces
Air replacement furnaces make the paint job cleaner because dirty air from the shop isn’t dragged into the booth. Air replacement furnaces make the paint dry faster because the solvent quickly leaves the finish to join the fast moving heated air.
The effect of the heated air on dry times can be illustrated with the blow drier in your bathroom. Try drying your hair with the switch set to "cool." The water on your hair will still evaporate into the moving air stream but it will take a while. Flip the switch to "hot" and see how quickly the water leaps off your hair into the heated air. The solvent leaves the paint film just like the water leaves your hair. Fast-moving hot air literally sucks the solvent out of the finish.
Air replacement furnaces also speed up production because paintwork can be forced cured at high temperatures to minimize the total solvent flash time. Heating the substrate not only helps drive off the solvents but promotes the cross-linking of the catalyst with the paint resins.
You can add an air replacement furnace to a crossdraft booth, too. This would make your work cleaner than sucking replacement air across the shop floor first. It would also make the paint dry and cure faster.
The style of heater in the air replacement furnace can affect the cost to operate the booth by thousands of dollars over the years. "Shops that only ask the price to buy the booth and don’t consider the price to operate it don’t have a true picture," says a sales manager for a major booth manufacturer.
The two choices for heaters in an air replacement furnace are direct-fired and indirect-fired units. A direct-fired unit is basically a big tin can with burner holes along the bottom for the fuel gas. Indirect units use a heat exchanger, which means they use the fuel gas to heat a core and then blow air across the hot core to create heat. Each style has its merits.
To get an accurate picture, the shop should add the cost to purchase the furnace to the cost to operate it in both the spray and bake cycles over time. The local utility company can provide the cost per BTU and electric kV in your area. Using the consumption specifications from the furnace, the cycle times from the booth manufacturer, the purchase price and a pocket calculator will show you the the cost to own and operate the booth.
A related cost is the life of the expensive ceiling filters. The cleaner the incoming heated air, the longer the filters will last. Look for air replacement furnaces with good pre-filtration and clean-burning characteristics.
Another cost that varies from design to design is the amount of air exhausted during the non-spraying times. All spraybooths exhaust 100 percent of the air during the spray cycle — that’s how they get the flammable solvents out of the way.
But exactly how much air is exhausted during the cure cycle varies by manufacturer. Electrical codes now require the air be removed by a fan during the cure cycle. Some booths simply run their main fan at slower speeds, some run the fan full speed against a damper and some use a separate fan. It helps to keep the air moving quickly even during the bake cycle because the remaining solvent still wants to get out of the film and into the hot, moving air. The cure cycle must remove at least 10 percent of the air every minute; the booth that can keep using more of the already heated air will be cheaper to operate.
Ask your vendor to help you calculate the energy costs associated with a particular design. Some methods can be thousands of dollars less at the end of the year, and it would benefit you to find out which ones.
Power Baking and Accelerated Curing
Many booth manufacturers offer a simplified power bake or accelerated cure mode. Rather than set all the variables of purge time, exhaust percentage, cure temperature and cure time at each use, they pre-set the unit to quickly dump in the heat with the push of a single button. Some units can even accommodate several painters, with a pre-set, one-button program for each.
Long term, it helps if the painters understand how all the furnace variables actually operate. Spray conditions change over the course of the year as the weather changes, different painters spray differently and different paints react differently. Setting the unit once and never touching it again is unrealistic. And having to call the far-off vendor for lengthy instructions is inconvenient. Get the information up front and write it down so the next new painter can understand and make necessary adjustments.
Air Movement vs. Speed
When comparing spraybooth specifications, don’t get too wrapped up in the volume of air movement. Concern yourself with air speed instead.
There’s no industry standard about rating exhaust-fan volume. Some fans are quoted at free air speed and some under various static resistance. For example, a fan with 15,000-cfm free air may only deliver 10,000 cfm at real-world static pressure. Look instead for the speed of the air.
With the car in the booth, the downdraft air should move somewhere between 100 to 125 feet per minute (fpm) in the envelope of air directly adjacent to the car. Without the car, the air speed may drop to 40 to 50 fpm in an empty booth. The further from the car you measure, the slower the air moves. Measure air speed about waist high, 5 to 10 inches out from the car. If you measure by the floor grate, the air is rocketing past. But what matters is the air speed right next to the paint film. More is better, until the air is moving so fast that you can’t buy solvents and catalysts slow enough to compensate for the drying speed.
Seeing Is Believing
That leads to my final recommendation: Go see the booth you intend to buy in operation in a real body shop, preferably one that’s spraying your brand of paint. Even if you just intend to modify your existing booth, find out from the vendor who has a booth with similar equipment.
Also bear in mind that every spraybooth vendor is sure they have the best answer to the problem. On paper every unique feature has its benefit. More air, less air, bag filters, panel filters, center pit, "H" pit, fluorescent lights and high intensity lights all sound great. Each vendor assures you their product is just what you need.
To be sure you’re making the right decisions, take the time and spend the money to travel to a shop that has the booth you’re considering buying. If the shop owner you visit likes the booth but wasn’t expecting to change filters as often as required or thinks the cure cycle takes longer than he expected, you can profit from his experience. It may still be exactly the booth you want, but now you’ll know to factor in more frequent filter changes or higher operating costs to get your true cost.
Ask around about the vendor who’ll deliver and assemble your booth, too. Good reputations are earned and hard to come by. If the vendor made other installations quickly, accurately and legally, they can do the same for you.
You Get What You Pay For
When it comes to purchasing a spraybooth, it pays to buy the best features. Buying a cheaper booth is likely to be more expensive in the long run: The cheapest cabin will spring leaks at the seams, the inexpensive lights will make painting more difficult and the low-line air replacement furnaces are likely to be noisy and eat fuel.
"What many body shop owners don’t realize is that the spraybooth, more than any other tool in the body shop, dictates the profitability and overall success of the business," says one spraybooth manufacturer’s marketing director.
Spend the money to buy high-quality, high-productivity, low-maintenance equipment and enjoy your investment every day. It isn’t just the initial cost that matters; it’s the number of cars you can paint that will make you the money.
Writer Mark Clark, owner of Professional PBE Systems in Waterloo, Iowa, is a well-known industry speaker and consultant. He’s been a contributing editor to BodyShop Business since 1988.
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