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Gunning for Perfection

Adapted from articles in BodyShop Business

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Today’s consumer is more critical than ever, so you’ve got to deliver the perfect color match and a flawless finish. To do that, it’s crucial that your spray gun performs as it was designed to. So what constitutes good gun setup? Correct gun setup and application techniques (both operator driven) control both transfer efficiency (TE) and finish quality. Get the setup 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.

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Setting the Gun
Some industry experts have completed extensive testing with rigid protocols to determine the best procedure for high-volume, low-pressure (HVLP) gun set up. They recommend that you begin by setting the fluid and pattern adjustments about 75% open, i.e. close the adjusting screws until no travel is possible. Then count the number of turns to reach wide open, and then close the screws 25% of the distance.

Begin by adjusting the pattern size. Change the fan width until the pattern suits the part being painted. (You may find that in some shops, some auto painters open the fan pattern adjustment to wide open regardless of whether they were painting semi trailers or side mirrors.)

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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.

Now Stop. Raising the air pressure any more will cause more turbulence in front of the gun and will affect both TE and finish quality. Want to know the range of inlet pressure to atomize well? Read the instructions!

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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.

A Little More on the Subject
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.

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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.

Top 3 HVLP Gun Problems
Speaking to leading spray gun manufacturers for this story, we learned that there can be some gun issues you young’ens need to be mindful of.

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  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.

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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.

  • 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.)

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    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.

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    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.

  • 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.
  • Did Ya’ll Know
    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.

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    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 around fluid tips and cause problems. Good, careful gun cleaning was never more important than with HVLP and high TE spray guns.

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