Painting Q&A: Choosing Air Spray, Air-Assist and Airless Spray Technology

Q. How do I choose the right spray technology for my application?

A. When purchasing liquid spray equipment, choosing the best technology for your application is key. Because there are many factors to take into consideration, it helps to understand the differences between air spray, airless or air-assisted airless technologies.  

Air spray uses a low-pressure fluid stream mixed with compressed air at the aircap to atomize material in a controlled manner. It’s used for the application of low- to medium-viscosity fluids for products requiring a high-quality, Class A or decorative finish.

There are three options for supplying fluid to an air spray gun: pressure-feed guns, gravity-feed guns and suction-feed guns. Pressure-feed guns are usually fed through a hose from a low-pressure pump or a pressure tank, and are used in industrial applications to move large amounts of fluid through the system. Gravity-feed guns have a cup mounted above the centerline to enhance the fluid flow using gravity and are suited for jobs using smaller quantities of fluid, such as touch-ups or repairs. Suction-feed guns have a cup mounted below the front of the gun and are used for handling small amounts of fluid, frequent small-volume color changes and low production rates. The suction is created by air flowing from the aircap, which draws fluid from the cup to the fluid nozzle.

Airless spraying uses a high-pressure fluid supply for atomization without the use of compressed air. It’s used for medium- to high-viscosity fluids, delivers a lower finish quality, and is useful for speed and transfer efficiency.

Airless spray atomization is created by hydraulic force pushing material through an orifice. As the fluid exits, friction between the fluid stream and atmosphere disrupts  into small particles. The tip size and pressure determine the flow rate. High pressure creates a complete pattern—the higher the material viscosity, the more pressure required.

Air-assisted airless uses a high-pressure fluid for atomization and compressed air at the cap for pattern control. It sprays medium to high viscosity fluids for a finish quality better than airless, but not as quality as air spray.

Air-assisted airless spray atomization is created by hydraulic force so it atomizes the center of the pattern. A fluid tip determines the pattern size and fluid flow rate. Air fills out the pattern, and an aircap completes the pattern, eliminating tails. Air-assisted airless solves many problems with the use of high-viscosity and high-solids coatings, and issues associated with heating and using higher fluid pressures for the atomization of viscous materials. Many waterborne materials require the higher fluid pressure of air-assisted airless because the lower pressure and the air-assist create finer atomization for a finer finish than airless.

There are advantages and disadvantages to each spray technology. Air spray is best for products requiring a high-quality finish, but doesn’t have a good transfer efficiency rate. Airless has a higher transfer efficiency rate and is often used for protective, rather than decorative coatings. Air-assisted airless offers high production levels and a high-quality finish. To identify the best technology for your application, determine your line requirements.

Wendy Hartley is a product marketing manager for electrostatic applicators, conventional applicators and spray packages at Graco Inc. For more information, visit graco.com/finishing.

Originally published in the March 2016 issue.