There are a variety of application methods to consider when coating a part or structure, from simple brushes and rollers to various spray methods. The method of application chosen can be based on a variety of factors and conditions. This article focuses on two spray methods: conventional (air) spray and airless spray.
Conventional (Air) Spray
Conventional or air atomized spraying uses clean, dry compressed air to transfer paint from a pressurized pot into a material hose (called pot pressure) and to atomize the stream of paint as it leaves the gun nozzle (called atomization pressure). There are two hoses from the spray pan to the Spray Gun. One contains the atomizing air (typically 50-75 psi) and the other contains the paint (at about 25-30 psi under pressure). The gun itself has two controls. One controls the amount of paint that comes out of the nozzle and the other controls the shape of the spray pattern. Many regular spray pans come with a stirrer in case the product needs to be stirred throughout the application process. The correct gun distance (from the ground) is generally considered to be 6-8 inches. Operators can choose from a variety of nozzle/needle assemblies and air cap configurations depending on the material being sprayed and the desired finish. The combination of these features provides the operator with greater control. This can be advantageous when applying coatings to small parts or complex structures. However, transfer efficiency (the amount of paint leaving the gun to reach the intended surface) is relatively low for conventional spraying, and dry spraying is common because the coating is atomized by compressed air. Also, due to relatively low atomization and tank pressures, viscous coatings may need to be thinned for proper atomization, potentially resulting in the release of more volatile organic compounds (VOCs) into the atmosphere.
High Volume Low Pressure (HVLP) spraying is very similar to conventional (air) spraying, except that it is applied at a lower pressure (typically 10 psi or less) at a higher air volume (hole count and hole size) There is significantly more air around the spray nozzle and in the air cap for atomizing the coating, so a greater volume of air is delivered.
airless spray
Airless spraying is the process of pushing paint at high pressure (say 3000 psi) through a small orifice (say 0.023 inches) and then releasing the pressure back to atmospheric pressure as the paint exits the nozzle. Pressure differentials (high to low) can cause coating fogging. Pressure is applied to the coating using a reciprocating pump that runs on compressed air, electricity, or fuel (gasoline). Airless spraying is a common spraying method on industrial structures.
Since the Spray Gun itself has no fan adjustment, the shape of the fan, as well as the amount of paint, are controlled by the nozzle. Airless nozzles are accurate to one thousandth of an inch (0.001 inch). A 0.023" nib produces more material than a 0.017" nib. The size of the fan is also controlled by the tip. Airless nozzles are coded by fan size and hole size. The fan width can be determined to be 12 inches from the surface by doubling the first digit of the nozzle code. For example, a 517 nozzle will produce a 10" fan, while a 523 nozzle will produce a 10" fan. However, because the orifice is larger, more material will come out of the 523 tip. A 719 tip will produce a 14" spray fan, while the 319 tip will produce a 6" spray fan. Even with the ability to control the width of the spray fan, the operator sacrifices some control compared to conventional spray. The correct Spray Gun distance (from the ground) is generally considered to be 12- 18 in. Transfer efficiency is higher than traditional spraying (no billowing of air leaving the gun with the paint), chances of dry spraying are reduced but not eliminated. Viscous paints are easier to atomize without thinning however, the atomization tail at the end of the spray pattern ( Poor atomized coating) is common in airless spray and can be corrected by changing pressure, increasing thinner, or replacing worn nozzles. Transfer efficiency is higher than conventional spraying (no billowing of air leaving the gun with paint), Chances of dry spraying are reduced but not eliminated. Viscous paints are easier to atomize without thinning However, atomization tails at the end of the spray pattern (poorly atomized coatings) are common with airless spray and can be corrected by varying pressure, increasing thinning can be corrected with spray or replacing worn nozzles. Transfer efficiency is higher than conventional spraying (no billowing of air leaving the gun with paint), chances of dry spraying are reduced but not eliminated. Viscous paint is easier to atomize without thinning however, spray pattern Atomization tails at the ends (poor atomization coating) are common in airless spray and can be corrected by changing the pressure, increasing thinner, or replacing a worn tip.
In summary, both conventional (air) spray and airless spray have many advantages and limitations. Conventional (aka pressure cooker) spraying is good for smaller projects with tight, intricate structures and uses less viscous paints, while airless spraying is good for larger, less complex surfaces where high production is the main goal. Skilled applicators can be used successfully. Paint manufacturers list suggested equipment and pressures on product data sheets (PDS) to suggest the method to use. PDSs provide great sources of information, especially when working with unfamiliar material.
Air-assisted airless spraying offers greater control over the quality of the coating, but since it uses air to help atomize the coating as it exits the gun, clean, dry compressed air is required.
