The pop saying "We've come a long way, baby," captures the progress the automotive paint industry. Ford was founded in 1903, GM in 1908 and Toyota and Volkswagen in 1937. In just more than 100 years of automotive production, applying color to these vehicles has evolved much. At first, cars were painted by brush. Although the spray gun was invented in the late 1800s, it was used primarily in the furniture business until the introduction of nitrocellulose lacquer by DuPont following World War I. The introduction of spray guns made it necessary to contain the overspray from this new type of application, which led to exhaust canopies — big fans that drew overspray from around the vehicles being painted. Later. the paint booth was developed, primarily to protect other people in the shops from overspray. However, in early pictures, painters spraying the cars rarely used respirators or other protective equipment.
So we have come a long way; the paint booth is now arguably one of the most important tools in the collision repair paint department. It has evolved from its original purposes into a tool that not only protects workers, but also cuts down on contamination of the environment, keeps the work clean and speeds up application and curing — thus increasing shop production. Though a significant investment, a paint booth that is properly maintained and operated efficiently can produce 10 or more paint jobs per day.
However, such progress has brought much controversy as well. Many laws mandate how we must operate these spray environments, what type of protective clothing operators should use and even the quality of air filters and how often they are changed. The most significant law or regulation is 40 CFR Part 40 Subpart HHHHHH, commonly called the EPA H6 Rule. A summary of the rule, which we’ll discuss here, can be found at ABRN.com/EPAH6.
Spray booths
Booth technology has changed as paint has changed, and paint booth manufacturers have developed equipment to meet the needs. Some of the first booths used — when lacquer was the primary coating being applied — were the crossdraft type (Fig 1), generally a simple non-insulated box that had filters at both ends and commonly worked by drawing air in from the rear and exhausting it at the front. Booths of this type worked principally to contain overspray and exhaust it to the outside. Lacquer paint primarily dried through evaporation, and a crossdraft booth helped speed up the curing time, though evaporation time before the paint could be buffed (necessary with lacquer paint) was generally 18 to 24 hours!
While these booths were a significant improvement over spraying vehicles in the shop and performed the task they were principally designed for — protecting workers outside the spray area from overspray — the crossdraft booths had some significant problems. They drew in massive amounts of air through the rear door, which in the wintertime could significantly lower the shop’s temperature and tax the building’s heating capability. They also drew massive amounts of air, contaminated with shop debris, across un-cured paint, often depositing dirt as the filters became filled. Filter technology at this time was crude at best, drawing air through paper arresters (filters). The openings in these paper filters were offset, causing the air to change direction, thus depositing paint mist on the paper as it passed by. Needless to say, most paint fumes were exhausted out the paint stack, and as incoming air quickly contaminated the rear booth filters, it allowed most of the dirt and debris to pass through into the booth. All vehicles painted with lacquer were polished to create the desired shine, so small dirt particles were eliminated during the polishing.
At this time, booths were beginning to be equipped with fans that forced air into the booth, and with a second set of fans that drew out the air. Because of the addition of a fan forcing air into the booths, the air flow could be adjusted by forcing in a slightly larger amount of air; then the exhausting fan would draw it out, thus providing a positive air flow in the booths. With more air coming in than going out, if there was a “leak” in the booth, it would blow air out rather than suck air (and dirt) in. Gauges (Fig 2) were installed on the booths that monitored air flow changes as filters became clogged. These air balance gauges (manometers) helped painters maintain proper positive air flow in booths throughout the filter life.
The next generation of paint booths was the downdraft group (Fig 3). These booths had incoming filters that covered the complete ceiling, and exhaust filters in the floor that directed the air straight down, passing over a much smaller part of the vehicle. The incoming air, on some, was pre-filtered before it was passed on to the ceiling filter, which eliminated nearly all the dirt and debris before it entered the booth. By pre-filtering the incoming air, the ceiling filter, which had increased in filtering efficiency to the point of eliminating particles as small as 3 microns, would last much longer. Filters in the floor, the first place paint is filtered, had also become more efficient and could filter out most of the dirt and paint particles before it is passed into the exhaust filters.
With the increase of air filtration efficiency and airflow monitoring, it was now more critical that painters control their booths more precisely and regularly. The introduction of basecoat/clearcoat paint decreased the need for polishing. Painters who wanted cleaner application of paints could no longer just wet down the floor before painting and expect to have a clean paint job. Booth maintenance was now much more critical.
There is now a third type of spray booth available— the semi-downdraft (Fig 4) — which is a hybrid of the cross draft and downdraft booths. It is a bit simpler to install and may be a less expensive.
Filters
Filters come in many different types, configurations and price ranges. They can be very inefficient, such as paper arresters or some fiberglass filters, which only trap large particles and quickly become clogged. Though denser filters cost more, they are also more efficient and trap more particles, allowing other more expensive filters to last longer. Pre-filters and exhaust filters are generally made from the denser filter media and in a “pillow” configuration (Fig 5). This allows for a larger surface area, making this configuration markedly more efficient. Ceiling filters are the most dense and costly; for this reason, many booths now use pre-filters to clean air before it is passed through the ceiling filter, extending its life significantly. With filters, a more expensive filter often lasts much longer and does a better job of keeping the vehicle clean.
The national standard for air filters under the H6 rule requires "shops that spray coatings containing targeted hazardous air pollutants (HAP)” to conduct spray operations in a booth outfitted with an exhaust filter having a removal efficiency of 98 percent or greater. Facilities should work with the spray booth manufacturer or filter supplier to select the right filter for the shop's filtration needs.
Changing frequency
A question the painter may ask is, “When should I change my filters? Is it monthly, or every five paint jobs, or when they look dirty?” The answer lies with the booth air balance meter (manometer, Fig 2). As filters become clogged, the flow of air through them becomes more difficult, and when this happens the booth’s balance changes; eventually the booth can no longer be balanced. On the booth control panel, along with controls for the temperature and time, is a balance control knob (Fig 6), which is used to balance the booth in both positive and negative directions. When the booth can no longer be balanced, filters should be checked. Start first with the floor filters; if, after they have been changed, the booth will not balance, the exhaust and pre-filters should be checked. Ceiling filters are changed less frequently, but should still be periodically checked. If all the filters have been changed except for the ceiling filter and the booth will not balance, then the ceiling filter too should be changed. A filter change-out list should be kept, showing when and by whom the filter was changed.
Some filters, such as floor filters, can be put in upside down. If a filter has one side that is open and less dense with the reverse side very dense, the open side should be placed so the overspray will hit it first. This will clean out the bigger particles, and as the overspray continues through the filter, it will clean the smaller particles. The egg carton shape of the filter in Fig 7 is facing up in the spray booth floor, thus trapping the big particles first. If placed upside down, the filter will become clogged very rapidly.
Don’t determine the need for filter changes just by how a filter looks. Often a fresh white filter in the floor will look dark and dirty after a black vehicle has been refinished. Also, the precise amount of airflow in a booth should be checked regularly. The booth’s airflow should be nearly the same at all four corners of the vehicle. The airflow of a loaded booth will be higher than the airflow of an empty booth. Knowing the exact amount of airflow in the booth is critical when applying waterborne paint. A significantly higher amount of airflow is needed to evaporate waterborne basecoat than solvent-borne paints.
Figure 7
Spray booth operations
As spray booths have become more sophisticated, their operation has become more critical. If operating properly, booths deliver safe and clean applications, as well as fast purging and curing of paint systems. So what should a painter do to reap the benefits of a modern booth?
First of all, the booth must be keep as clean as possible. To do this, never open booth doors unless the fans are running and balanced. That is, the booth should be in a positive air flow, as monitored by its booth air balance meter or manometer gauge. When a booth is balanced with positive airflow, it is less likely to draw dirt and debris in when doors are opened. If the booth is balanced with a negative airflow or the booth is off, when the doors open, dust (which is statically charged) will enter the booth; and when a vehicle, which is positively charged, is placed inside, the dust will be drawn to the vehicle.
In the past, painters would often wet the booth floor, place a vehicle in the booth and ground it by draping a chain over the axle and letting the other end be grounded on the wet floor. This practice was eventually abandoned because it caused booth floors to rust. With new downdraft booths, water on the floor quickly evaporates during the painting process; so when the painter applies the clear, a critical point for dust, there is no wet floor benefit.
Other practices that help keep a booth clean include the application of a sprayable masking on the booth’s walls that can cleaned off and reapplied as needed, as well as covering the floor with paper (Fig 8), which is changed as it gets dirty. And never sand or mask in the booth — all the prep work should be done in a prep station or using a vacuum, as far away from the paint booth as possible. As much masking as possible should be done before the vehicle is loaded in the booth, and the booth should be cleaned before each paint job (Fig 9). Though controversial to some, experts recommend a balanced, generally clean booth be blown before a vehicle is loaded. The air stream of the blow gun is directed toward the floor filters, so it can be trapped there, and not left to contaminate the vehicle while painting.
The masked vehicle should be blown off with high-pressure air before it is loaded into the booth. The complete vehicle should be blown off, including the masking paper and all cracks where dust can hide. Then a chemical cleaner is used just before the booth doors are opened and the vehicle loaded. Many painters believe that the vehicle should be pushed in instead of driven in, so as not to contaminate the booth with exhaust. This is wise, especially with diesel-powered vehicles.
After the vehicle is in the booth, the doors closed, the balance checked, and the finish masking completed, the vehicle should then be blown off again with low pressure. The surface area to be painted should be cleaned with a final cleaner, most of which now contain an anti-static component such as alcohol, to help eliminate any static that may have built during preparation. Finally, just before spraying, the vehicle is tacked. (The tacking process is a good time for the technician to give the vehicle a final inspection before spraying.) An often overlooked tip for keeping a vehicle clean while it is being painted is to limit access to the booth. That is, no one without a paint suit should be allowed in, and the least number of trips into the booth as possible should be made by the painter. It is best to mix all the coatings that will be needed, take them into the booth, and paint the vehicle. The fewer times the door is opened, the less likely dirt is allowed to enter. If the booth is equipped with a mixing room, no one should enter the booth except through the mixing room access door. Booths equipped with a vestibule (a small room that painters go into to clean up and suit up before going into the mixing room) significantly reduce dirt in the paint. Painted floors, which are easier to clean, also cut down on dirt in paint.
Operating a productive and profitable paint shop that keeps its employees safe and complies with all the regulations that are now required can be complex, but not necessarily time consuming. An efficient shop will develop a paint maintenance standard operating procedure that can be completed regularly. Whether the booth maintenance is done by the painter or assigned to a painter’s helper, the results will be well worth the time spent.
Figure 9
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