If you run your spray gun off a compressor, you know you have an infinite range of pressure you can use. Too little pressure, however, and you don’t get the best atomization — you get orange peel. Too much pressure, and you waste material because of excessive bounce-back.
How do you determine the ideal pressure?
Some spray-gun and finish manufacturers provide a suggested air pressure, and you may find this works fine for you. But there are many variables manufacturers can’t take into account, including the actual stain or finish you’re spraying, how much thinner you’ve added, the length of your air hose, and temperature variations (liquids become thicker in cooler temperatures and require more pressure to atomize).
In addition, manufacturer-suggested pressures often don’t specify whether they are measured at the compressor’s regulator, the gun’s inlet or at the air cap.
In order to adjust the pressure at the inlet or air cap, you need an air gauge that attaches to these locations. For example, if the suggested pressure is measured at the air cap (where the HVLP requirement of 10 psi or less is measured), you require a special cap with an attached gauge — for an extra couple hundred dollars.
You don’t need any of these gauges, however, and you don’t need to rely on manufacturers’ suggestions. You can figure out the optimum air pressure for your gun and for the finish you’re spraying with just the regulator and a simple test.
To find the optimum air pressure, begin by opening all the controls on the gun to their maximum and turning the air pressure at the regulator down to well below where you think it should be — for example, to 20 psi. (Regulators are attached to smaller compressors and are mounted on the wall with larger compressors that use piping to one or more locations.)
Then spray a short burst onto brown paper or cardboard. (The finish shows up better on a brown surface than on white paper.) You’ll get a relatively narrow width pattern with noticeably large dots around the edges.
Increase the air pressure by 5 or 10 psi and spray another burst. The pattern will be a little wider and the dots a little smaller.
Continue increasing the air pressure in increments of 5 or 10 psi and spraying short bursts. Each time you increase the pressure, the pattern will get wider and the dots at the edges of the pattern will get smaller.
It’s important to hold the gun at the same distance from the target for each burst. The easy way to do this is to open your hand fully, placing the tip of your little finger against the target and the tip of your thumb against the air cap on the gun. Then spray each burst at this distance, which is about 8 inches.
When you reach a pressure that doesn’t widen the pattern from the previous and doesn’t make the dots smaller, you’ve gone too far. You have achieved the best atomization, but you’re now wasting material because more than necessary is bouncing off the target.
So reduce the air pressure to the previous setting or maybe a little further, to just before the pattern starts shrinking and the dots start becoming larger.
This is the optimum setting for the viscosity of the material you are spraying in the current weather conditions. As Jerry Hund, the former education director at Binks, used to say, “When the pattern is right, the pressure is right.”
Changes require retest
As long as the viscosity and weather conditions remain the same, there’s no reason to do the test again. Simply set the air pressure at the regulator the same each time you spray.
If you change to a different finish material, or if you thin it differently, or if the temperature changes (for example, if you keep your shop cooler at night, the finish will be thicker first thing in the morning), you’ll need to perform the test again to find the optimum pressure.
Once you have established the optimum pressure, you can narrow the fan width a good bit without losing significant efficiency.
This test doesn’t work with turbine-air supplied guns because you don’t have the same control of air pressure.
Bob Flexner is author of the book “Understanding Wood Finishing.”
This article originally appeared in the November 2008 issue.