For those in the paint trade, one of the most controversial subjects that's been discussed and argued throughout the years is spray technique. Which is the best gun, paint, spray gun set-up, gun adjustment and reduction viscosity? Which is better, painting from the bottom up or from the top down? It's a topic about which painters have strong opinions.
Yet the discussion continues because of a common goal: to produce the best results in the most efficient and profitable manner. Let's look at what the industry considers best practices in spray techniques for solvent-borne basecoats and clears.
Spray gun
The guns are external-mix spray equipment, which is important to know, because almost all the force that's used to break the liquid paint into small atomized droplets happens outside the gun within a tiny amount of time and space. In Figure 2, the liquid paint, represented by the dots, is pushed through the nozzle by a small amount of air, while the much larger amount of air is diverted through the spray horns to be used as the force that atomizes the paint before it hits the vehicle. This is important to understand because if the amount of paint that comes out of the guns nozzle is too large, it can't be broken down into the small droplets necessary for a smooth, orange peel-free paint job.
With the introduction of High Volume Low Pressure (HVLP) spray equipment, which reduced the amount of overspray often produced by older equipment, the needle nozzle setup was less important because the air pressure could be increased to high amounts. In the past, sometimes 70 PSI was used for spraying lacquer, and large amounts of overspray were produced (Fig 3). When painters use HVLP equipment, the guns reduce the amount of overspray (which is less pollutant) and use fewer materials. These advantages are possible because the HVLP gun's transfer efficiency can be higher than 65 percent compared to 2 percent with older guns. However, to achieve these results, the needle nozzle setup has become more critical.
The spray gun company can give you only a range of setups, such as 1.2 to 1.6, as is often seen. With this information, many painters have purchased guns with the middle 1.4 needle nozzle setup, thinking it would be OK to spray different types of coatings. More accurate information would come from the paint manufacturers, who are more likely to have a recommendation of a specific needle nozzle setup and paint type. A painter armed with this knowledge increases the likelihood of the gun spraying at its top efficiency and quality.
Being this specific about how to set up a gun may require a painter have multiple guns set up for specific paints or clear – even a gun set up specifically for sealer. Also, because HVLP guns have specific air volume requirements, the older half-inch hoses aren't sufficient to supply the air flow needed to allow an HVLP gun to work properly. Figure 4 illustrates the inside diameter of the quick-change fitting on the left is significantly smaller and won't supply an HVLP gun with enough air volume for it to run at peak performance. Though both fittings can be attached to a 3/8-inch hose, the one on the right will allow the correct amount of air to be supplied.
Critical factors
Distance, overlap and flow rate are critical factors, and techniques in these areas must be learned for a painter to become efficient and profitable.
The distance the gun is held from the object being painted, the amount of overlap each consecutive spray pass is applied, and the movement rate are critical to a good spray paint application. Remember, if the external mixing gun is held too close to the surface being painted, the paint won't have the time and air volume to break the droplets down into the fine atomized particles required for a smooth follow out. Also, if the gun is held too far away, the fine droplets will start to reform as larger droplets, and a more textured, dry appearance will result.
Authorities have determined the correct distance is 5.5 to 6.6 inches from the surface. In the 1990s, considerable work was done to develop a laser-aiming device that would train painters to reach and maintain this ideal spray distance. As illustrated in Figure 5, if the round dot shows two dots, it's too far away or too close to the object, and the longer one shows the width of the overlap being painted. Though these devices did a good job helping new painters become better at their application techniques. The laser aiming device never caught on widely in training facilities. Additionally, a virtual reality spray trainer was developed for the Army to train repair technicians in how to apply the new classified radar-absorbing coatings. However, because of their expense, neither of these trainers have been adopted widely in training facilities.
Overlap is the amount of paint that crosses over the previously applied stroke of paint. In years past, before the use of coatings that contained metallic, mica, pearls, and other so-called glamour additives, a 50-percent overlap was sufficient. In fact, the DeVilbiss paint company had a highly used diagram depicting the 50-percent overlap as the industry standard. But with the introduction of HVLP guns and the use of high metallic and other additives, the 50-percent overlap is no longer good enough. A 75-percent overlap is now the standard recommendation. This requires more strokes per panel to paint the same amount of space, but with the 75-percent overlap, it's less likely that the dreaded streaking and mottling will occur.
Rate of travel indicates how fast the gun is moved along the vehicle's surface. The proper rate is about one foot per second. But most painters judge their flow rate by watching the wet line, which is done by viewing the area being painted at a 45-degree angle. Under most conditions, this isn't a difficult task, and as the new coat of paint is being applied, it can be viewed through the stroke application. In Figure 6, the painter stands back from where the clear is being applied, to view the amount of overlap and how fast the clear is being applied to the basecoat. Under most conditions, this will be easy to see. But when applying clear to a white car, viewing the wet line can be difficult. When encountering this type of condition, a painter must judge how well the clear is being applied and should inspect for dry areas and touch up the panel as needed at the end of painting each panel before moving on to the next.
Lead and lag
Lead is the amount of paint sprayed before the target is hit. Lag is the amount of paint that continues after the target has been passed. Figure 7 shows a large lead and lag paint technique which, though it might not affect the quality of the paint work, significantly affects the amount of paint materials consumed. If the lead and lag is kept to a minimum, as shown in Figure 8, the amount of materials the painter uses will be reduced significantly.
Fanning is the movement as the painter changes the angle of the spray gun at the end of each stroke (Fig. 9). The spray gun must be held perpendicular to the surface of the vehicle as it's sprayed, or it will change the color of basecoat or the texture of the clear significantly. Pitch is the expression for the condition when the gun isn't held perpendicular to the panel being painted. If the angle is closer on top, the paint will be heavier there and lighter at the bottom. Known as towing the gun, this can happen easily as the length of the painter's reach is nearing its fullest. By towing the gun, a painter often ends up with the paint being streaked. By holding the gun closer at the bottom of the fan (heeling), streaking also can occur. This was more common when heavy siphon cup guns were used. A gravity fed gun with smaller 600ml cups and cup balancing has helped a lot with these two pitch problems.
Plan of attack
Each paint job needs to have a plan formulated for the best sequence in which a vehicle should be painted. Start with the front and go back? Work from the top and paint down, or paint from the bottom up? Often the sequence is almost predestined because of the number and placement of the panels. Whether a painter should paint from the center of a panel and work out or go around the panel and paint from the edge is logical. The wet line is critical to the quality of the paint work, so if a painter is spraying a hood, it should be started at the outer edge near the fender, and he should work inward toward the center as far as he can reach comfortably. Then the painter should move to the other side and paint from the center outward toward the hood's edge. If it were started in the center and he worked to the edge, by the time the painter returned to the center of the hood, it may have dried enough that a visible line would be in the center that could only be removed by detailing.
Top down or bottom up?
The simple answer to this question is vehicles should be painted in the direction of the air flow. If you're painting in a downdraft paint booth, the vehicle should be painted from the top down. This keeps any debris that might be dislodged while painting from falling into a wet area. If the vehicle is being painted in a cross draft paint booth, it should be painted in the area where the air comes into the booth first and where the air is pulled out of the booth last.
The payoff
Learning to control all the variables to paint a vehicle can be challenging. Understanding why they affect painters and the work also is challenging. But if a consistent spray technique can be developed and the control of the all the aforementioned variables is accomplished, painters can become efficient and profitable for themselves and their companies.
