SPRAY GUNS 101

SPRAY GUNS 101
Learning the basics of spraygun operation can help keep the paint flowing--and the shop running--smoothly.

By CHARLIE BARONE, Contributing Editor
January 1999

Paint technicians striving to boost their levels of performance must heed at least one piece of advice--learn the operating principles behind the sprayguns they use each day. Getting back to the basics also teaches them how to avoid paint failures and how to keep spray equipment in top working order.

A spraygun's mission is to atomize refinishing material--it takes paint droplets and divides them into thousands of tiny particles. At the heart of this is the nozzle set, which consists of the air cap, the fluid tip and the paint needle, says David Bond of SATA Spray Equipment. The size of the individual particles depends on surface tension and the viscosity of the fluid stream, and the compressed air at the nozzle prevents the fluid from exiting the tip in a continuous stream.

"Refinish quality will only be as good as the atomization [the gun] provides and the skill of the painter," say Bond and Tony Larimer, also of SATA Spray Equipment. "When the gun is triggered slightly, it opens the air valve, and the spraygun acts as a blowgun. As the trigger is retracted further, it unseats the needle in the fluid tip and material begins to flow. On a gravity gun, the quantity of fluid leaving the gun is controlled by the viscosity of the material, the size of the fluid tip and the needle adjustment. Normally, this adjustment should be nearly fully open, thus allowing full trigger control. On sprayguns where the material is pressurized fluid, flow is also affected by air pressure."

Setting the Equipment

Working hand-in-hand with atomization is proper gun setup. "spraygun set up should be the best combination of the proper nozzle choice, pressure setting, fluid flow and fan width," Bond and Larimer say. "For conventional guns, the general rule of thumb is to use the lowest pressure that will provide the best atomization and spray pattern."

Most sprayguns allow technicians to adjust fluid flow and spray pattern, and many products feature built-in air micrometers to vary the pressure setting. If this is not an option, a separate air micrometer may be used. The fan control is used to control the spray pattern width. As a rule, most painters want to use as wide a fan as possible, but as the fan widens, the amount of material distributed decreases.

To remedy a large-droplet problem, use more air pressure or adjust the fluid needle to reduce material flow. Adjust material flow and droplet size to provide even material distribution across the entire pattern.

Spraying with HVLP guns is another matter, however. With this type of gun, only 10 psi is available to atomize the material. Many HVLP guns use an integrated air converter, so after selecting the proper nozzle and setting the fluid control, follow the manufacturers' recommended inlet pressure to set the air cap pressure at 10 psi. The HVLP gun may be fine-tuned for maximum performance by slowly increasing the inlet pressure at the air inlet and varying the fan control. This can be accomplished by using an accessory air cap with dual gauges to check center and horn air cap pressures. Because many new paint products contain low-VOC, high-solids material, maximum atomization and performance from the gun are essential. Remember that at-omization becomes more difficult as the amount of material increases. If you need help selecting nozzle size, consult your supplier or the paint manufacturer.

Technique Essentials

After setting your equipment properly, it's--as they say--all in the wrist. Even with the spray equipment and the optimum gun settings, a technician's technique can ruin a paint job. "[An] improper spraying technique will result in uneven coatings, mottling, runs and orange peel," Bond and Larimer say. "Murphy's Law really applies here."

To improve their jobs' finish quality, painters must control:

* the distance from the spraygun to the target;

* spray pattern overlap;

* triggering;

* working speed;

* spraygun angle (heeling, toeing and fanning).

Painters' spray techniques are also overwhelmingly dependent on constant visual feedback: They have to watch what they are doing. Be sure to apply the paint evenly--pass the gun perpendicular to the surface at a constant speed. Slowing the rate of the pass will put more material on the panel, and increasing speed will decrease the amount of material applied. Use the speed to regulate your material, and watch the panel very closely to see if you are "wetting" the surface. Each pass of the spraygun should overlap the previous one by about 50 percent. Watch the material closely to see how it is being laid down. (HVLP guns are better for this because of the decreased amount of overspray and better visibility.)

Never go over the same surface twice. It is always better to let the material flash and then correct the problem on the following coat. Depending on your selection of reducer and hardener, the material will flow to some extent after it has been applied. But once it has "kicked"--the point at which the material begins to catalyze--what you see is what you get.

Don't be shy about applying the coatings. Painters who never get runs just aren't trying. If a painter is afraid to "wet" the panel, he or she will produce dry-looking paint jobs with excess orange peel. Remember that runs in the clearcoat are usually repairable after they have cured. Most painters would rather sand out a run and reshoot the panel than have an entire orange peel job. On the other hand, don't rush the job or try to fix a defect by piling on material. That almost always compounds the problem.

Follow directions to prevent such mishaps as paint application failures and runs because of insufficient flash times. If the successive coats are landing on material that hasn't had the chance to evaporate its solvent, you'll end up with a river. It is better to err on the side of caution. If you are in doubt regarding flash time--whether it's because of low or uneven temperatures on the surface of the car--allow an extra few minutes to let the solvent evaporate. This is often the case with crossflow spraybooths, where parts of the car's body are colder than others. The cold side will tend to run before the warmer portion.

If debris or insects land in the finish, a steady hand and a pair of tweezers can often save the job. The topcoat will flow out to some extent if the material hasn't cured, leaving you with an imperfection that can be sanded and polished to perfection. In more severe cases, you will have to respray the panel. When in doubt, wait for the finish to dry.

Bond and Larimer also offer these troubleshooting tips:

* Spitting or paint bubbling in the cup is caused by atomized air getting into the fluid passage. This is usually the result of insufficient tightening of the fluid tip or a loose air cap. It can also be caused by damage between the flat surface of the fluid tip and the fluid tip spindle.

* Spray pattern fluttering can be caused by too little paint in the cup, a loose fluid tip or a damaged needle seal.

* A "sickle" or quarter-moon pattern is usually caused by partially or completely clogged air horn passages.

* Too much air pressure may cause a split pattern. Remedy this by increasing the fluid flow and narrowing the fan pattern.

* A teardrop pattern is the result of dried material on the outside of the fluid tip and a bent needle or fluid tip.

What's Your Preference?

The siphon-feed design has been around for what seems like forever, and in the United States, it is the most popular spraygun configuration. But in Europe, the gravity gun is used more often. Painters there have grown up with its advantages--air supply isn't used to pull the fluid to the cap, so it requires lower pressure to do the same job as its siphon-fed counterpart. The one drawback to the gravity gun is its inability to be placed on any flat surface--technicians need a special bracket or hook. But a growing number of painters believe the gravity gun is better balanced than siphon equipment because the majority of its weight is positioned below painters' wrists as they work. Extended roof panel painting causes fatigue when using the siphon gun, but the gravity gun is less stressful for your muscles.

"The typical automotive refinish shop will generally utilize three types of hand-held sprayguns: gravity-, siphon- and pressure-feed," Bond says. "Most manufacturers produce these variations in both HVLP and conventional versions. In past years, the siphon gun was king, but gravity guns have become more and more accepted and have replaced siphon guns mainly due to the new paint technology. Sales figures from our company have proven this to be true. Our company sells virtually no siphon guns in Europe."

Although these recommendations target beginners, revisiting the basics can help more experienced painters, too. Young paint technicians should also remember that practice is the best teacher, and they should begin enhancing their skills by applying primer-surfacer because that coating will usually be sanded. Remember to review the safety precautions noted on the MSDS, and above all, relax and have fun. When the equipment, material and techniques are in sync, painting is a lot of fun.

Ask Doctor Gun

Hub Fornsgren, Sharpe Manufacturing's in-house spraygun expert, often conducts clinics on spraygun operation and maintenance. Also known as "Doctor Gun," he hosts a question-and-answer forum on the Sharpe web site (http://www.sharpe1.com) and offers these bits of advice:

ABRN: What are your recommendations for cleaning equipment in an enclosed gun washer? More and more facilities are using these to comply with the law.

Dr. Gun: Remember, these are supposed to be working in 60-second cycles. Never leave the gun in there longer. Everyone has a favorite gun they like to use. What happens to the others? They often wind up in that gun washer. This is like submerging the spraygun in lacquer thinner, which is something you should never do. If the solvent is not changed regularly in the gun washer, condensation begins to form, which also causes acids to form. That's harmful for a spraygun. The solids in the gun cleaner solvent are also harmful. Continual use of dirty solvent will cause deposits to form in the spray equipment, similar to varnish in an old carburetor. Another thing to remember is to keep the solvent out of the air passages. Plug the air inlet of a gravity gun before placing it in the gun washer to prevent the solvent from getting inside when the gun is inverted.

ABRN: And after the gun is clean?

Dr. Gun: Remove it immediately and dry it with a cloth. Never let a gun drip dry, even though the solvent evaporates quickly. Change the solvent regularly.

ABRN: What about the packing?

Dr. Gun: Packing should be changed as leaks develop. However, you can use a gun indefinitely and never have the gun apart or change the packing.

ABRN: If my spraygun's pattern is bad, do I have to rebuild the gun?

Dr. Gun: If a spraygun has a bad pattern, a rebuild kit is not necessarily going to affect the pattern. The tip and cap are the only things that affect the pattern. You have to balance the air with the material to avoid delivering too much material.

ABRN: Is the widest pattern always best?

Dr. Gun: No. When applying clears, a wide pattern will do, but pay attention to the pattern. Have you ever seen one that looks like a football? That's not good. The spray pattern should be even from top to bottom. Open wide makes for more overspray.

ABRN: What do the holes on either side of the air cap do?

Dr. Gun: Those are the air horns, and they act like an air anvil. They flatten out the spray to make a pattern. The side holes (on either side of the fluid needle) are to define the air pattern. The impingement zone is where air meets material. When air comes out of the horns, it will give you air out of each side that hits directly in front of the air cap, like a thumb over a water hose.

ABRN: On a gravity gun, will the fluid rate decrease as material mass diminishes?

Dr. Gun: On some of our gravity feeds, we incorporate a siphon assist, which pulls along the fluid stream while it is being pushed out. It keeps the overspray away. As material decreases in the cup, the gun needs this assist.

ABRN: Can I still use one gun for most refinish materials?

Dr. Gun: That's not likely if one is to stay in compliance. The inlet pressure is critical to maintain 10 psi at the cap, and this does not allow increasing the atomizing air to compensate for higher viscosity materials. If you are not in a compliant area, don't worry about the 10 psi at the cap, and increase the inlet pressure. The maximum inlet pressure marked on most HVLP sprayguns is in reference to the pressure at the cap--a maximum of 50 psi equals 10 psi at the cap.

ABRN: Must painters change their technique when using HVLP sprayguns?