The balancing act of spray gun atomization
By Marty Schlosser
April 13, 2021 | 1:29 pm CDT

Photo By Fuji Spray Systems

So what exactly is atomization and what does it do?

Atomization is the breaking down of the fluid stream, using pressurized air, into fine droplets which are then projected onto the target surface.

Air cap set selection
It all begins with mounting the correct air cap set -- consisting of a fluid nozzle, needle and air cap – for the finish you’re planning to use. Simply stated, the higher the viscosity (think thicker…) the larger the nozzle needed to pass that finish through the hole. And because that fluid stream will be larger, your air cap also needs to be larger to deliver more air to break up that larger, heavier fluid stream.

You could refer to a chart, but realistically speaking, for most applications you require only one of three sets: 1.0 for dye or shellac; 1.3 for sanding sealer or topcoats, including lacquer, varnish or polyurethane; and 1.8 for anything of higher viscosity, such as tinted lacquer or latex paint. If you have an unpressurized gravity gun, move up to the next larger set for all finishes.

The balancing act of atomizationWith the optimal air cap set mounted and your gun clean and properly lubricated, strain the finish as you load your gun or pressure pot. Don’t overfill the reservoir; about 3/4 is right. And don’t forget to put on a protective mask and goggles for safety.

Test spray
Start by setting the gun to full air, widest possible fan and the fluid control knob opened two full turns from the full off position. Orient the air cap to deliver a vertical fan pattern (air cap horns horizontal). Note that the manual that came with your system will indicate where those various controls are located and how to adjust them. Then set up a board for test spraying, which will allow you to see how things are proceeding as you adjust your gun. I use cardboard box scraps cut to 2-foot squares, which I fasten to the wall of my booth.

Holding your gun 7 inches from the test board, the standard distance for spraying, and ensuring it is perpendicular to the surface both horizontally and vertically, squeeze the trigger for a brief second or two. Don’t move the gun along as you normally would when spraying; aim it at the same place. Then take a critical look at your test board results.

First, you’re looking for any evidence of blockage in the air passages (the air diffuser or air passages inside the gun, or the air cap itself). The pattern should be oval-shaped. If it is comma or curved shaped, there is a blockage: one of your air cap horns, or the air diffuser or air passages needs to be cleaned. If it’s clean and there’s still a misshaped pattern, it may be that your needle or perhaps the nozzle has been damaged. Address the issue then conduct test sprays to confirm that it’s been corrected.

Looking again at the pattern, if it is hourglass-shaped, it most likely means that the air volume is too high and needs to be reduced. Make small incremental changes – nothing greater than 1/8 of a turn. Conversely, if the pattern is too heavy in the center, increase the air pressure, or, if you’re already fully open, it means the finish’s viscosity is too high (again, think thick) for the air cap set and needs to be thinned or the next larger air cap set installed. Continue performing test sprays and air volume adjustments until you achieve the desired pattern.

We’re at the point where we can see if the finish is being correctly atomized. What we’re looking for are very minute droplets hitting the test panel. These droplets will often be more visible towards the outer area of the oval pattern you sprayed. If the droplets are large or coarse, it means the ratio of fluid to air is too high: turn down the fluid knob 1/8 turn and perform another test spray. If they appear just about right, go the other way to see if it can be improved. Keep doing this until you are satisfied that the atomization is as fine as possible. Be willing to play around with the air volume as well, and only make small adjustments, 1/8 turns, between each spray test. And don’t try more than one setting at a time; either fluid or air, but not both, at least not until you’re confident you’ve got this in the bag.

With these steps completed, your gun is now properly set to deliver a finely atomized spray, at the widest possible fan pattern. But if what you’re spraying calls for a narrower fan or a circular pattern, or if you need to reduce the air pressure to work on the inside of a cabinet, you’ll have to make some changes. Adjust the fan width or air pressure – or both, in some cases – then go back and perform the atomization checks. In no time at all, these checks and adjustments will become second nature, allowing you to efficiently move from one pattern setting to another throughout the spraying session.

Source: Marty Schlosser is a longtime woodworker and shares his knowledge of finishing through his partnership with Fuji Spray Systems. For information on Fuji visit FujiSpraySystems.com or call 800-650-0930. Read more tips from Marty in the blog section on Fuji’s website.

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