Setting fluid and air pressures

When I visit plants for a tune up in their spray shops, I almost always notice one thing. Painters using air spray guns have their gun atomization air pressure turned up way too high. I often see it turned up as high as it will go. For most plants, that’s about 120 psi. This is especially true in shops with lots of painters and high turnover. Everybody wants high production rates, and that includes painters. Pulling the trigger with high air pressure gives the feel of power, speed, and high productivity. Who wouldn’t like that?

Before anyone gets addicted to high air pressures, let’s ask ourselves one question. Where does that air go when it leaves the gun?It goes into the air, because it’s air. But the goal of painting is to get paint onto the parts, not into the air. Too much air takes the paint that you want on the part and puts it into the air. More air pressure equals more overspray. Less air pressure equals less overspray, greater productivity, lower paint usage, lower VOC’s, and reduced filter usage.

So how do you set the air correctly on an air spray gun with a pressure pot?

1. Turn the gun air off
2. Set your fluid pressure at about 15 psi.
3. Pull the trigger with the gun held in a horizontal position.  You’ll get a steady stream of paint from the gun.  Ideally, the “pee stream” will go horizontally for about 8 – 10 inches before it starts to fall.  
4. If necessary, adjust the fluid pressure at the pot up or down until you achieve the 8 – 10 inch distance.  
5. Turn to atomization air to about 15 psi. If you have a pot with dual regulators, you’ll do this at the pot.  If you have a single regulator pot, you’ll do this at the air source leading to the gun.  
6. Pull the trigger for a quick ½ second burst onto a piece of paper or cardboard.  Observe the spray pattern.  If the pressure is too low, you will see “fingers” at the each end of the pattern.  
7. If necessary, increase the atomization air until the fingers disappear.  The result should be an oval pattern with evenly distributed spray.  
8. Do a “fast pass” onto the paper or cardboard.  Observe the particle size.  If the coating is well atomized, your air pressure is sufficient.  Do not increase the air past this level.  
9. If the fast pass has large particles, you can try these things to improve the atomization.   
   1. Thin the coating. 
   2. Increase atomization pressure. 
   3. Reduce the fluid pressure. 
   4. Change the air cap and fluid tip to the correct size for the intended coating. 

Remember, more air pressure isn’t necessarily good. In fact, it’s usually bad. I recommend that paint shops keep records of the best fluid pressures, air pressures, and appropriate amounts of thinner for each coating that they spray. It would be helpful if painters would follow the recommended guidelines. When there are too many cooks in the kitchen with no recipe, the air pressures usually go up and they stay up. That wastes a lot of time, paint, and money. Fortunately, this situation can be easily avoided.