Print

Email

It was an unbelievable 15 years ago that the South Coast Air Quality Management District (SCAQMD) passed its now-famous Rule 1151. In an effort to clean up the devastating air pollution in Southern California, the rule addressed the spray equipment used within its jurisdiction.

Calling for 65 percent or better transfer efficiency (TE), the choices were limited. Powder coating has an extremely high TE but has trouble applying metallic finishes, not to mention the very expensive equipment and sterile environment needed for successful powder application and baking. Electrostatic painting can exceed the 65 percent TE they wanted, but auto painting poses a difficult challenge. The parts of the auto are made from disparate materials: aluminum, plastic, steel and fiberglass. Gaskets and sealers insulate steel parts - which could carry an electrical charge successfully from one another but don't because they're insulated - so electrostatic also has trouble with metallics. Almost by process of elimination, you needed to use high-volume, low-pressure (HVLP) spray equipment to comply with Rule 1151.

But California wasn't the only state cleaning up its act. Many other states and regulatory areas enacted legislation of their own to comply with the Clean Air Act of 1990, which called for reducing air pollution by 15 percent.

Today, there's still confusion over spray guns that are HVLP and those that are "compliant." If your lawmakers call for HVLP spray equipment, the test to determine what you need is twofold: The gun should have 65 percent (or better) TE and pass no more than 10-psi air pressure at the air cap. To my knowledge, no regulation anywhere defines high volume. Low pressure is 10 psi or less at the cap. Several regulated areas, which call only for 65 percent TE, can use guns that are compliant but not HVLP. The main difference between HVLP and new "compliant" guns is the ability to exceed 10 psi at the air cap.

Let's take a closer look at both HVLP guns and these new high-transfer (but not HVLP) "compliant" guns, along with examining common HVLP guns problems and how to make the most of your gun - regardless of which type you choose to buy.

TE and Conventional Guns
A recent study by the Iowa Waste Reduction Center (IWRC) at the University of Northern Iowa discovered that similar transfer efficiency is available from well-set-up, conventional spray guns. They conducted a controlled comparison between a suction-feed (cup on the bottom) conventional spray gun and gravity-feed (cup on top) HVLP gun. The siphon gun had the long-time industry standard - .070 fluid tip (1.8mm) - and the HVLP had a 1.4 (.055 inch) fluid tip. (A useful key: 1/16 inch = .0625 = 1.59mm).

Two sets of identical truck hoods and fenders were primed and sanded, and the painter was a professional autobody painter who used the siphon-feed gun daily. The base color and clearcoat were mixed to exact paint-manufacturer instructions.

The painter applied two coats of basecoat and two coats of clear to each set of panels, using a different gun for each set. Each gun with paint was exactly weighed to within one-tenth of a gram, both before and after spraying. They tested carefully for film build and subjectively for finish quality.

Their findings were surprising.

• The HVLP gun used 13 percent less basecoat color, but the conventional gun used 12 percent less clearcoat.
• The gloss and smoothness of all four panels were judged comparable and acceptable.
• The film build on the hoods was quite similar, but the HVLP gun put 33 percent more clear film build on the fender than the conventional gun. This no doubt contributed to the greater amount of clear the HVLP gun used.
• If the HVLP gun was at 65 percent TE, then the conventional gun was at 57 percent TE. Not the huge advantage we often hear about for HVLP equipment.

So why, then, have HVLP spray guns saved auto painters thousands of gallons of paint and thousands of dollars in material costs? The IWRC concluded that HVLP's reputation for higher TE is likely the result of lowering the ceiling for excessive set-up parameters and good operator training. (On HVLP, you can't get so far out of whack.) On conventional spray guns, the typical (male) auto painter opens the fluid passage, fan control and air pressure controls to full-boat, wide open immediately. More must be faster and better right? But HVLP guns are limited by the 10-psi rule, and you can't atomize the paint smoothly if the fluid passage is wide open at only 10-psi air pressure.

What we're learning here is that the most important variable in getting high TE isn't the gun, the target or the paint. It's the operator. This is why properly training your painters on gun set up and spray technique will make the biggest difference in TE.

Got Air?
If your shop has struggled to get good paint jobs with HVLP spray equipment, a likely problem is the volume of the compressed air required. Although no regulated area defines "high volume," each spray gun has a requirement for air volume that must be met. If you starve any brand of spray gun for air, it won't work very well.

First, remember that this issue isn't about air pressure. It's about air volume. Measured in cubic feet per minute (CFM), it describes how much air volume the gun consumes while spraying. How much air does your brand require? I don't know. If you don't know, we've got a problem. Some high-transfer spray guns require 16 to 18 CFM to work right. Others require 9 to 11 CFM, and some little jamb guns need only 2 CFM. The spray gun manufacturer is the only place you'll find out how many CFM of air are required for a good job using your brand and model spray gun. Ask your vendor or read the catalog.

You can, however, roughly calculate how many CFM of air you have available for the gun. Each one horsepower on the electric motor that drives your air compressor should produce approximately 4 CFM of air. On smaller compressors (less than 5 hp), the formula is for every one horsepower, you should expect 3 to 31/2 CFM. So, your 10-hp paint shop compressor should provide about 40 CFM (10 hp X 4 CFM = 40 CFM).

Even if your brand of HVLP spray gun requires 20 CFM to work correctly, you're OK, right? Right, but É only if no one else is using much compressed air. Start up a couple of dual-action sanders (5 to 10 CFM each) a 5-inch grinder (10 to 20 CFM each), and there's nothing left for the paint gun.

Small-sized air pipe contributes to this problem in many shops. Even with 40 CFM available, much of the volume is literally scrubbed off as the air squeezes through narrow (1/2-inch or 3/4-inch) air lines on the way to the spray gun. Bigger diameter shop piping is money well spent.

The issue of air pressure also leaves some long-time painters confused. With either HVLP or new high-transfer-compliant spray guns, the required pressures are much lower than the 65 psi many of us learned to spray acrylic enamel at. With very high inlet pressures, the gun had a "kick" when you pulled the trigger, and the escaping air had a satisfying "hiss" as it rushed out of the air cap. Low inlet pressures (10 to 30 psi) have neither the kick nor the hiss and must be very accurately adjusted to get the best results. Not only can't you do it by feel or sound, you must have accurate and functioning regulators to get it exactly right.

For those with HVLP guns in regulated areas, you must not exceed certain inlet pressures to ensure that the outlet pressure at the air cap never exceeds the 10-psi limit. What are those upper limit inlet pressures? Only your paint gun manufacturer has the answer. Read the literature carefully to find out how much air you can put in and still stay under 10 psi out. Substantial fines in some areas and poor finish quality in all areas can result from improper gun pressures.

Setting Up Your Gun
What constitutes good gun setup? Remember that correct gun setup and application techniques (both operator driven) control TE and finish quality. Get it right and the gun will do most of the work; get it wrong, and you'll spend the whole paint job trying to overcome the defects inherent in poor set up and technique.

The IWRC did extensive testing with rigid protocols to determine the best procedure. They say to begin by setting the fluid and pattern adjustments about 75 percent open, i.e. close the adjusting screws until no travel is possible, count the number of turns to reach wide open and then close the screws 25 percent of the distance.

Begin by adjusting the pattern size first. Change the fan width until the pattern suits the part being painted. They found most auto painters opened the fan pattern adjustment to wide open (surprise!) regardless of whether they were painting semi trailers or side mirrors.

Once the pattern is set, adjust the fluid flow (move the needle in or out) until a workable amount of paint is delivered to the fluid tip. Wide open allows so much fluid out that to atomize it, the air pressure must be cranked way up, which ruins TE by blowing the paint off target.

With the pattern size and fluid flow properly set, slowly raise the air pressure until the droplets are correctly atomized. Stop. Raising the air pressure more will cause more turbulence in front of the gun and will affect both TE and finish quality. What's the range of inlet pressure to atomize well? Read the instructions!

Remember to set the air pressure to the paint gun using recommended working pressures. That means pull the trigger and adjust the air pressure while air is flowing freely through the wall regulator and the spray gun. Once you let go of the gun trigger, the pressure behind the gauge will climb. Don't worry. When you pull the trigger to begin painting, the pressure will fall to the correctly set working pressure.

Other pertinent set-up issues include the fluid tip diameter and the solvent speed. Backward as it sounds, new high-solids clears will work better when squished through a smaller fluid tip. I know. It seems wrong. Shouldn't higher-solids material work better by going through a bigger hole? Nope. High-solids clears are hard to atomize. The more clear escaping, the more air required to break it up. By keeping the fluid tip small (1.2, 1.3, 1.4mm,) the job of breaking it up is easier, especially if the gun is limited to 10-psi maximum cap pressure.

Solvent speed also plays a major role in finish quality. Several experts agree you should never use fast-dry solvents with HVLP or high-transfer spray guns. You need at least a medium speed or slower solvent to flow out the finish. The longer the solvent stays inside the paint film, the smoother it will be. The extra moments (note moments, not minutes) required to let a slow solvent flash off are returned many fold in less buffing and leveling time.

The Top 3 HVLP Gun Problems
I polled five leading spray gun manufacturers for this story, and one of the questions I asked was, "What are the most frequent HVLP problems from the field?"

1. Cleanliness. One thing that hasn't changed with new paint solids and new gun efficiencies is that the painter is still in a hurry! No time to clean!

Two developments have particularly affected the clean-up process. One is the gun design. In an effort to get the highest TE and best finish quality, several gun manufacturers have added more small holes to their air caps or fluid tips. So dried paint or clear more easily plugs these very small holes.

The other change that's exacerbated the clean-up problem is the high-solids clears. Once it dries inside the small holes in the spray gun, the clear is hard as a rock. And dried clear doesn't dissolve readily when the gun is soaked in cheap solvent. It's also hard to see since the light still passes through the clear, so the passage looks clean.

These two factors have caused an increase in gun problems due to nothing more serious than poor cleaning. Gun manufacturers recommend the painter take more care during cleaning. Using a better grade of solvent than the recycled lacquer thinner that many shops use also would help.

Remember, the only part of the gun that has paint in it is the very end. Soaking the cap (air nozzle) and tip (fluid nozzle) in rich solvent will do a better job of cleaning. The issue of the rock-hard dried clear, however, will require a change in procedure.

For years, every paint gun manufacturer cautioned against using wire or metal to clean out holes in the air cap or fluid tip. They called instead for wooden match sticks or broom straws. But the new clears are so hard that wood won't do the job in many cases.

The gun manufacturers still caution their customers that careless use of a metal probe will gouge and enlarge the holes in brass or aluminum parts easily. Be careful! Still, the rigid metal probe will do a better job of getting the dried clear out of small holes.

Hardened clear can even build up unseen (it's clear!) around fluid tips and cause problems. Good, careful gun cleaning was never more important than with HVLP and high TE spray guns.

2. Pulsating or spitting. This happens because the vacuum is being broken at the very end of the gun.

Every air cap has three purposes: suction, atomization and pattern containment. The suction action (even on gravity-feed guns) is possible because the crossed streams of air cause a partial vacuum in front of the gun. Paint is pulled into the vacuum and atomized. Pulsating is caused when some gun component isn't tight and extraneous air is leaking in, spoiling the vacuum. (Spit, spit, spit.)

The most likely cause is the connection between the fluid tip and the spray head. The fluid tip may or may not have a crushable gasket under it and may or may not require Teflon tape on the threads.

Whatever method your gun manufacturer uses to ensure the joint between the two parts is airtight must remain intact. If you've removed the fluid tip to clean it (good for you), make sure it's airtight when replaced. Sometimes this means tightening the fluid tip to the gun body with a vise and a ratchet. Some brands use thread sealer to ensure no leaks. Find out the correct procedure for your brand before you overtighten a tip or clog the passage with unwanted thread tape.

After a loose fluid tip, needle packing is the next most likely cause of spitting. As the fluid needle moves back and forth, it wears the packing and air can leak in and ruin the partial vacuum. Tighten the packing crush nut according to manufacturer's instructions.

3. Bent or cracked fluid tips. If you knock the spray gun off the paint bench and onto the floor, especially if the air cap is removed, the precision end of the fluid tip is very vulnerable. In my experience, most slightly bent tips are put back into service. Hey, a new part is $60 to $100. But even a slight distortion will negatively affect how the gun sprays. I'm sorry you accidentally dropped the gun. Be more careful, but bite the bullet and buy a new, undamaged fluid tip.

Something to Get Excited About
I asked the gun manufacturers what's new and received some interesting answers.

• One manufacturer is introducing detachable spray heads. The complete spray assembly, air cap, fluid tip and fluid needle come off as one part. And they offer a rack to hold the sets not in use. You can switch from a 1.9mm fluid tip and needle for primer to a 1.4mm tip/needle for color and a 1.3mm combination for HS clear - with no tools required.

• Another manufacturer has LCD digital pressure gauges built into the handle of some guns. Precise and accurate psi readings will contribute to getting the exact match on difficult metallic colors.
• One brand is offering bolt-on colored identifiers to quickly locate a gun hanging on a rack with many others that all look alike. All you have to do is find your gun with the yellow or red tab attached to it.
• One brand has a quick-change air cap that comes off with one turn.
• 4Another brand has a unique shape to its air pattern to retain the same pattern size regardless of distance to the target.
• Several manufacturers have guns suitable for waterbased paints (must be rust-proof inside). They suggest that you dedicate a gun to waterbase rather than switching back and forth from water to solvent paints throughout the workday. And since they clean up with different chemistries, the waste streams can be kept separate as well.
• Perhaps the most significant new development is the advent of the new high-transfer, but not HVLP, guns from several manufacturers. They all predict that the ability to raise the air cap pressures will improve atomization. As you know, base color shoots very well with HVLP equipment - 10 psi seems to be plenty. However, as the solid levels climb in clearcoats, they become more and more difficult to atomize and breakup. More atomizing air, along with possible regulatory approval, make these new high TE guns worth a try if your locale allows them.
• One new trend common to several spray gun manufacturers is to anodize a colorful design onto the gun body. Whether flames, checkered flags, American flags or color-changing finishes, these guns look cool! I tried to find out how it was done and was told the process is "all proprietary information" and not available to us mortals. Many of these showy guns never see paint inside them, but I know of at least two in daily use with no deterioration to the snazzy design. Bear in mind that some paint removers and all abrasive pads will discolor even the best anodizing if you scrub hard enough.

Oh Yea … and Read the Directions
So what's my parting shot on today's HVLP guns? Read the directions that came with the gun. Gun manufacturers have spent plenty of time and money designing their products. Take the trouble to read what they suggest you do to get the best results.

Good painters can accommodate many paint gun failings - from low atomization pressures, to too fast a solvent, to lopsided spray patterns. But many times their job-saving tricks require another coat to smooth out the finish. Get the gun adjusted right, keep it clean and follow the directions, and you won't need that last extra coat - saving labor time, the environment and material costs.

Clean up, rebuild and understand your current gun, whether it be HVLP, high-TE compliant or conventional. They'll all work better.