Lest you doubt that technology in our business moves right along, consider the sophisticated chemistry that makes up the new VOC-compliant products. It was just a few years ago that predictions about the impending National Rule were dire indeed. “Look out,” we said, “the compliant, low-solvent products will be thick, viscous and difficult to use.” As we now know, these predictions weren’t true, and the products are not only environmentally friendly, but also user friendly.
Advances in paint chemistry have been remarkable; advances in paint gun technology are no less amazing.
What have these advances been? What type of spray guns are on the market today? How do you decide which ones to use in your shop?
All good questions. Keep reading, and you’ll find your answers.
HVLP and Granny’s Vacuum Cleaner
The idea of a high volume of air at a low pressure (HVLP) has been around since the 1940s. When the door-to-door vacuum cleaner salesman convinced your grandmother to buy his snazzy new electric vacuum, he also offered as an option a spray gun that would connect to the vacuum cleaner. By turning the motor on the vacuum to blow rather than suck, you could spray paint with the optional spray gun. It’s unlikely your grandmother was terribly interested in going from vacuuming the parlor to painting the car, however, so the equipment was hardly commonplace.
Such equipment didn’t come to the forefront of our attention until the South Coast Air Quality Management District (SCAQMD) mandated that everyone painting with solvent-based finishes in air-quality-challenged Southern California apply them with high-transfer equipment. Although powder coating is 100 percent transfer efficient and electrostatic equipment can exceed the mandated 65 percent transfer efficiency (TE) as well, neither lend themselves to auto refinishing; both methods have trouble with metallic reflectants and struggle to deliver the “Class A” finish required on cars. Almost by a process of elimination, HVLP spray guns became the method of choice for Southern California auto refinishers.
As other states strived to enact legislation to reduce air pollution by 15 percent to comply with the Clean Air Act of 1990, HVLP spray guns spread across the country. (To my knowledge, no one anywhere defines what constitutes “high volume.” Those early vacuum cleaners certainly produced a healthy volume of air. Measured in cubic feet per minute (CFM), that vacuum cleaner could produce 50-60 CFM. By comparison, a piston-driven body shop air compressor produces roughly 4 CFM per horsepower on the electric motor. By that reckoning, it would take a 15-horsepower shop compressor to produce 60 CFM of air. On the other hand, several states, California among them, do define what constitutes low pressure. It’s generally 10 pounds per square inch (PSI) or less at the air cap.)
Although they were gaining in popularity across the country, early HVLP spray guns left a lot to be desired. For example, they had vacuum-cleaner-sized air-supply hoses, and they often had no on/off valve — so air escaped out the front of these “bleeder type” guns continuously, which many painters didn’t like.
In addition to the physical changes these early guns required the painter to accommodate, they often left a finish with unacceptable orange peel. Orange peel is caused by droplets of paint remaining intact rather than melting into one another, which can be cured by some combination of better atomization (smaller droplets) and/or a slow evaporating solvent (melts down flatter).
During these first few years, the HVLP equipment was more cumbersome and produced a less desirable finish than most painters preferred. In the following years, however, HVLP guns evolved. They began using regular shop air hoses and started looking and feeling more like the conventional siphon-feed guns the industry was accustomed to.
These days, spray-gun engineering has brought the entire industry to a new level of success. If your shop hasn’t purchased new, high dollar, topcoat HVLP spray guns in the recent past, you’re in for a pleasant surprise: better atomization, even greater material savings and many ease-of-use features.
There’s More Than One Way to Build a Spray Gun
There are three ways to construct a traditional autobody paint gun, and each can incorporate HVLP technology.
1. Siphon- or suction-feed spray guns — These guns incorporate the principle that Dr. DeVilbiss discovered. DeVilbiss was an ear, nose and throat physician looking for ways to medicate his patients’ sore throats. If his patients simply drank the liquid medication, it passed quickly over their inflamed throats and disappeared into their stomachs. The atomizer DeVilbiss designed, however, broke the medicine into small, atomized particles, which remained in contact with his patients’ sore throats long enough to do some good.
For 40 years, the siphon-feed spray gun with a .070-inch-diameter fluid tip was the standard in our industry. But a basic flaw of any siphon-feed gun, including the HVLP versions, is the turbulent cloud of air directly in front of the air cap. Siphon feed works because the spray gun crosses two streams of air in front of the fluid tip. Directly behind the angle formed by the crossed streams (called the angle of impingement), a partial vacuum is formed. The breather hole in the top of any siphon-feed cup allows atmospheric pressure to enter. Atmospheric pressure is 14.7 PSI, while a partial vacuum is less, say 10 PSI. The liquid in the cup rushes up to fill the vacuum, and paint is sprayed out the front of the gun. But the crossed streams of air create turbulent, swirling air movement. Once the paint is sucked up to fill the vacuum, it’s released into the swirling air, which blows the paint off target. This cloud of air required to suck paint up to the fluid tip is why siphon-feed guns don’t have very good transfer efficiency.
2. Gravity-feed spray guns — In these guns, the cup is mounted over the gun and gravity forces the paint down into the atomizing air stream. Two air streams still cross in front of the fluid tip, but the angle of impingement is less and, as a result, the moving air is calmer. The carefully machined holes in the air cap do more than control how much air whistles around in front of the gun, however. An air cap on any gun has three purposes: to create suction, to atomize the fluid into small particles and to shoot air outward along the outside edge of the paint to contain and shape the pattern.
Gravity-feed HVLP spray guns have become the new standard of our industry. Even so, I hear painters objecting to two things. One is just plain silly, while the other has merit.
Upon first picking up a gravity-feed gun with the cup on top, the silly painter says, “I can’t see what I’m painting.”
Time out! Good painters don’t stand behind any spray gun because all you can see is a cloud of atomized paint. Good painters position themselves so they can see the leading edge of the pattern strike the surface. This is how you know the paint is flowing out and covering the target. Granted, the loaded paint cup on top of the gun feels different than the cup on the bottom, but regardless of the type of gun, you still have to stand to the side to see the paint strike the target.
The objection that has some merit is that gravity-feed guns must be hung from a hook rather than set down on any handy flat surface. This, however, is hardly insurmountable and a small price to pay for better transfer efficiency and substantial paint-material savings.
3. Pressure-feed spray guns rely on air pressure to push the paint up to the fluid tip. This is the highest possible transfer efficiency, and pressure guns with HVLP technology lead the pack for the most paint on the target and the least in the air.
Pressure-feed equipment has been around for decades but often had a 2-3-5-gallon pot to contain the paint and 25 feet of fluid hose connected to the gun itself — just the thing for painting buses or airplanes. You could hold the gun at any angle to reach any contour. In collision repair, however, as much as 75 percent of typical damage can be fixed with less than one pint of paint, but cleaning the pressure pot, hoses and gun each time would require gallons of solvent to wash out a few ounces of color.
Several manufacturers produce pressure-feed HVLP spray guns with a one-quart pressure pot attached directly to the gun. While this design has very high transfer efficiency, it also has a check valve. Check valves are one-way valves; they let something pass one way but not the other. The 2-3-5-gallon pots used to paint buses or trailers also have check valves to keep pressure on the paint to push it forward — and to prevent the paint from squirting back up the airline. The difference is, the 2-3-5-gallon pot remained stationary on the floor, but the one-quart pressure pot is waved around as the painter paints. As a result, the check valve often gets filled with paint and the gun ceases to function well.
Still, many paint shops use pressure-feed HVLP spray guns successfully every day. To do so, they keep their guns scrupulously clean and view proper gun maintenance as a vital part of success.
This Isn’t Texas. Bigger Isn’t Better
Not only have late-model HVLP spray guns gotten better at atomizing the finish (even the new high solids products), they’ve also improved their transfer efficiency. These days, there’s no shortage of manufacturers that sell really wonderful gravity- or pressure-feed HVLP spray guns. While there are many differences among brands, there are a couple of considerations I believe apply to all makes and models.
First, I suggest you choose spray guns with smaller-diameter fluid tips than you have in the past. In a business where labor time is king and everybody endeavors to move at the speed of light, bigger would seem better. The bigger the hole in the end of the spray gun (the fluid tip), the more material that comes out. But, the more material, the more air that’s required to effectively atomize it — and it’s the resultant cloud of turbulent air that ruins transfer efficiency. To make the problem worse, the new VOC-compliant finishes are higher in solids. Pushing paint with more resin and less solvent out of the same old, large-diameter fluid-tip hole makes the atomization problem even worse.
I was surprised, along with many others, to wake up one day to find the entire industry talking about the hole in the end of the fluid tip in millimeters rather than thousandths of an inch, as we had done for decades. When everyone was using siphon-feed guns, most were equipped with a .070-inch fluid tip and matching needle. Conventional pressure-feed guns had .0425- or .055-inch fluid tips because you could push more paint through the hole than you could pull paint through the hole with a partial vacuum. Now, it seems, we speak of the fluid-tip hole diameter in metric equivalents: .51 inch equals 1.3 mm and .070 inch equal 1.8 mm.
Whichever method you use, choosing smaller fluid tips will ensure better atomization and better transfer efficiency with the new high-solids products. Even if you spend a few more minutes waving the gun at the vehicle, your total time to complete the refinish for customer delivery will be less.
If you have a current model topcoat HVLP gun, make sure it has the fluid tip and needle combination that your paint manufacturer requires. How will you know? Ask them. Every paint company has written recommendations for fluid-tip diameters to ensure its products work up to their potential. Still, I frequently meet painters who can’t make some paint manufacturer’s product work to suit them. Later that same week, however, I’ll meet other painters who think that same product is so wonderful that they swear by it — rather than at it. What’s the difference? I don’t think it’s painter talent; painters from one productive shop are fairly similar to those in another. I think it’s equipment — and solvent choices — that shape their opinion toward a paint product.
Take a Gun for a Test Run
What spray gun do I recommend? I suggest you buy one that works best for your skill set, your spray conditions and your quality requirements. If you’re painting cast-iron widgets with flat, black, industrial enamel, for example, one of the first-generation turbine units will work best for you. If you’re painting a 1999 model hood, top and decklid, you’ll need a current HVLP gun.
On paper, every gun manufacturer has the best possible solution to your problem; it says so right in the literature. But instead of buying a gun from a catalog, take it for a test run first.
Begin by asking your paint rep what size tip and needle will work best for his company’s high-solids, compliant coatings. Next, approach your jobber with the sizes and have him recommend a brand of gun to try. Just like paint products loved by some and hated by others, believe me when I say I’ve seen literally every brand of HVLP gun do a good job in some circumstances. The brand your jobber supports with parts and service would be a great place to start.
Once you have a gun to test, you’ll need to adjust it — something that most painters don’t do. In our defense, most of us are men and, as such, we simply open every adjustment to wide open and get the hell going. That’s not necessarily smart, but it’s what we do.
I recommend you initially set the gun to 75 percent open. Holding the gun in one hand, screw the pattern-adjustment knob (usually the top knob) all the way in. Now, open it, counting the number of turns out till it stops. Turn it back in 25 percent of the turns; this sets the pattern size at 75 percent of its maximum.
Next, screw in the fluid control (generally the bottom knob) until there’s no needle movement when you pull the trigger. Now count the turns out until the trigger rests against the gun handle. (Continuing to turn the needle travel knob further out is fruitless at this point because the gun is already wide open when the trigger bangs against the handle.) Now, turn the adjustment knob back in 25 percent of the distance so the fluid flow is also set at 75 percent open.
To test for even distribution across the spray pattern, try this: Turn the horns of the air cap, which face east and west as you look at the gun, to face north and south. This will shoot a pattern horizontal to the ground rather than the vertical pattern used to paint. Shoot the test pattern on white masking paper at medium air pressure. With the pattern oriented across the masking paper, it’s much easier to see if the pattern shape is uniform. (Simply blasting a pattern on the edge of the masked car right before you begin doesn’t tell you much.) If the pattern is uneven, try exchanging the north air horn for the south (flip the air cap over). If the distortion in the pattern moves with your change, the problem lies in the air cap. If the problem remains in the same place, the fluid tip is likely damaged. The main cause of damaged fluid tips? Simply knocking the gun off the paint bench onto the floor. They don’t bounce very well.
With the gun set to at 75 percent open and the pattern checked, you can now follow this adjustment advice from the experts in transfer efficiency at the Iowa Waste Reduction Center: Adjust the pattern, then the fluid flow and finally the air pressure. First, reduce or enlarge the pattern to suit your target object. Don’t be afraid to change the pattern size frequently while painting to maximize transfer efficiency.
Once the pattern is sized appropriately, increase or decrease the fluid needle travel to adjust the paint-flow rates. A general tip for success with any HVLP spray gun is to slow the solvent speed considerably. Slower solvents stay in the paint film longer and allow the droplets of paint to flow smoothly into one another. Another general rule when using high-solids paint products is to lessen the needle travel, which restricts the amount of paint that can get through the hole. Just like smaller fluid tips lead to greater atomization, choking off the paint flow makes it easier to atomize, too.
Once the pattern size and fluid flow are set, simply adjust the air pressure until the droplets stop splattering and start flowing. This is another case where less is more — keep your air pressure at the minimum amount required to atomize the paint. More air pressure causes more overspray and less transfer efficiency.
Try the test gun on at least two jobs before you buy it because the first time you use any gun won’t produce your best results. Make every effort to keep the spray gun parallel to the surface being painted. Cocking your wrist or arcing the gun can triple the overspray in the air. Be as machine-like as possible when you wield the gun, but remember it’s the artist in you that makes your paint work look good.
By the end of the second or third job, you’ll know whether you like what’s happening. If the gun seems too slow, the finish too rough or the pattern too small, change guns and try again.
Painting is a very personal thing and there’s still no one right way to do it. If a gun feels cumbersome and wrong, keep looking. If, however, you’re coming directly from conventional siphon feed to late-model HVLP (where have you been anyway?), all the guns will take some getting used to. The upside is that you’ll save material and get cleaner results.
Ready, Aim, Paint!
We’ve come a long way since the days when spray guns were optional equipment for Granny’s vacuum cleaner. Imagine Grandma outside painting Grandpa’s car with a spray gun in her hand and a vacuum cleaner by her side!) Lucky for us, today’s HVLP spray guns are much more sophisticated than their predecessors.
Don’t have a late-model HVLP spray gun in your shop yet? What are you waiting for?
Frankly, there’s never been a better time to buy an HVLP spray gun than right now. Your paint work will be cleaner, your spray environment will be healthier and the money you save on material will pay for your investment very quickly.
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.
In Defense of Undercoats
Both topcoats and undercoats are very expensive these days, and many shops shoot their colors and clears through a top-quality, gravity- or pressure-feed HVLP gun. However, they’re often using old, leftover siphon-feed or low-price HVLP guns for their undercoats. While I think a $150 HVLP gravity-feed gun is better than an old, siphon-feed gun, I think the expensive topcoat guns will recoup their cost in undercoats, too.
I suggest you take your old HVLP topcoat gun and let it become your undercoat gun and then buy a new HVLP topcoat gun. “Whoa,” the shop owner says. “I coughed up $450 to buy that gun three years ago. Now I’m going to shoot primer or sealer through it?” Yup. Not only that, you’re going to come up with another $300 to $500 to buy the latest version HVLP topcoat gun. I contend the cost will quickly be recovered in material savings by using less paint and in labor savings by having smoother finishes.