Experts explain the differences between primer and paint spray guns.
We all know that automotive paint and primer have changed a lot in the last couple of decades with the serious reductions in VOC type solvents being mandated by the EPA. This has led to improvements in paint gun function, and given rise to new guns engineered for specific purposes, most notably one for primer and another for color and clearcoat. Professional shops have no problem with this, but too often do-it-yourselfers are resistant to purchasing two paint guns. Instead, many hobbyists just buy one spray gun, thinking that changing the tips and needles is all that’s needed. Considering that the cost of a pro-quality paint gun can easily be north of $600, this is understandable.
What must be understood is that, especially today, primer is not paint. Primer is the foundation and paint is the color; they are not the same. According to representatives from SATA, one of today’s leading spray gun manufacturers, modern primer and paint products are more sophisticated than ever and require more precise mixing and application. One gun will not do everything.
The good news is that new paint guns must meet the EPA standard of a 65% transfer efficiency rate, which means that more product ends up on the panel. On top of that, a primer-specific gun is designed to minimize overspray and put even more product on the panel to reduce the amount of sanding needed, meaning less primer ends up on the floor in the form of sanding dust. Because of this, it’s possible that the cost of a specific gun for primer could be paid for by the savings of less primer used and time saved because less sanding will be needed. Also, there will be less waste to have to dispose of. Because more primer ends up on the panel, there is less chance of sanding through and needing to re-prime the car again. Considering the overall cost of completing any project that involves paint, the purchase of a primer-specific gun is a small price to pay for what has the greatest impact on the look of your ride.
Muscle Car Restorations of Chippewa Falls, Wisconsin, has been producing award-winning paintjobs for decades. Among their keys to success is the careful attention MCR pays to new technologies and practices recommended by their spray gun manufacturer. MCR uses SATA guns exclusively, and a recent visit from Dan Am Company’s Director of Sales and Marketing, Tony Larimer, was informative.
As the HVLP designation implies, this primer gun flows a high volume of air, and therefore primer, at a low pressure—in this case, 10 psi or less at the cap. One of the main differences between primer and paint is that primer has a higher viscosity. This viscosity difference requires a different airflow pattern out of the cap to properly atomize it. When done properly, this leads to smoother coats and less waste. Think of it like an inkjet printer laying ink on paper: The smaller the droplets and the more closely they can be laid down, the smaller the gaps that are between them. This results in smoother coverage with less product needed to achieve it.
Compliant RP guns operate at slightly higher pressure at the cap, typically around 18 to 20 psi, reduced from the 29-psi inlet pressure. Their airflow patterns are optimized for lower-viscosity color products. Unlike primer guns, color guns must be designed to contend with paints that contain metallics and pearls in a way that avoids uneven patterns or blotchiness. Again, small droplets closely placed yield the smoothest coats and the best flow out of the gun.
Getting Down To The Fine Details Of Spray Gun Design
While these air caps (primer HVLP cap on the left) may look very similar, the hole sizes, placement, and angles, as well as the air chambers under them, are very different and have specific purposes beyond just the different flow rates designated by the 1.9 and 1.3 markings. In fact, SATA HVLP primer caps will not even fit on an RP color gun. It is important to note that caps, nozzles, and needles are a matched set. The way the gun sprays will change as they wear, especially with metallic colors, and they will have to be replaced as a set. The differences between an HVLP primer gun (left) and an RP color gun go beyond the nozzle size. One is specifically engineered to spray primer products and the other basecoats and clears. Simply swapping nozzles, needles, and caps does not change one into the other.
There is a lot of engineering built into a pro-level paint gun. The red area indicates where the paint flows. Blue areas are airflow. Note that there are multiple air channels flowing through the cap. The placement, angle, and size of each nozzle are far from random. Minor changes have a dramatic effect on the way the paint or primer atomizes, fans out, and is deposited on the panel.
Just when you are starting to warm up to the idea of a separate primer gun, look at these options. The black one is a compliant gun designed to spray high-build poly primers, texture coatings, or even bedliner materials. The smaller silver SATAminijet 4400 has a smaller fan spray designed for shooting jambs and other smaller areas, or small repairs. The gold one is SATA’s top-of-the-line gun, with pattern options. It can spray a traditional “I” pattern with an ideal distance of 6 to 8 inches or an “O” pattern for closer-in and faster painting.
Here you can see a definite difference between the air hole placements of “O” vs. “I” nozzles.
This is what a primer spray pattern is supposed to look like (left). Notice the wet center portion of the spray and the relatively narrow overspray band. Primer guns are designed to put maximum product on the panel. A wide overspray band is not needed, as primer does not have the blending issues of the base color-coats, especially since it is usually sanded once dry. However, color patterns (right) do require a broad overspray band around the wet center to ensure proper blending, especially across the large panels and when metallics and pearls are in use.
It’s not just the gun but also the appropriate nozzle size for the materials that are to be sprayed that achieves good results, as well as the paint itself and the condition in which it is sprayed. SATA’s director of marketing, Tony Larimar, strongly recommends that a local paint distributor be used to help select the proper nozzle for the specific paint products being used, and for the local climate to be considered, as temperature and humidity conditions are big factors in the success of any paint project.