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Prepare a vertical test
Vertical panels expose fluid-control errors immediately because gravity acts on the wet film before the coating can build enough viscosity to hold its position. Runs usually begin where overlap is excessive, gun speed slows, the pass changes direction, or the painter adds material to compensate for weak atomization.
Start with a spray-out card mounted vertically. Mix and strain the coating, then record panel temperature, reducer speed, nozzle size, and viscosity. Set dynamic pressure with the trigger fully open. Open the fan to a uniform shape and begin with the fluid needle about one and a quarter to one and a half turns out.
Establish movement and overlap
Use the lvlp spray gun at a consistent distance of roughly 150 to 180 mm, adjusting only if the coating manufacturer specifies otherwise. Hold the air cap square to the surface. Begin each pass beyond the panel edge, reach full material flow while moving, and release fluid before the opposite edge. This prevents heavy deposits at the turnaround points.
Control film build with travel speed and fluid output, not by varying distance. For a first test, use approximately 70 percent overlap and a steady forearm movement. If the lower edge of each pass becomes darker or wetter, increase speed slightly or close the fluid control by one-eighth turn. Avoid raising pressure until the pattern has been inspected.
Control film build by zones
Divide the panel into upper, middle, and lower zones. Spray the upper zone first, then work downward so overspray settles onto an unsprayed area rather than contaminating a finished wet film. Maintain the same gun speed across styling lines, handle recesses, and panel crowns. Slow hand movement in these areas is a major source of vertical sagging.
Use side lighting to identify the moment the surface changes from evenly wet to overfilled. A smooth reflection with no moving bead is the target. If a bead begins to travel, stop adding material and allow the section to flash. Do not chase the run with extra air because this can create edge mapping and uneven gloss.
Read the wet film
An air spray gun may atomize at a different pressure and air volume, but vertical-film control still depends on measured output and disciplined overlap. Compare ten-second material output between setups rather than copying knob positions.
With the lvlp spray gun, complete three vertical test sections at one-eighth-turn fluid intervals. Mark the section that provides full coverage without downward movement after two minutes. Record pressure, fluid position, distance, speed, overlap, and flash time.
Lock the production setting
During vehicle application, reduce fluid slightly for narrow pillars and panel edges, then return to the validated setting for broad areas. Keep the hose behind the body so it does not pull the gun off angle. This procedure creates a stable wet film, prevents top-to-bottom flow accumulation, and reduces sanding and repainting caused by runs.
For clearcoat, verify the first coat before increasing fluid for the second. The second coat should improve flow without doubling wet-film thickness at overlaps. Use a timer for flash control, and never rely only on how glossy the panel appears under booth lamps.
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