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Paint flowing down at the bottom of vertical parts is usually a film-build control failure, not just a material problem. The lower section of a door, quarter panel, bumper side, or rocker cover often receives extra paint because the operator slows down at the end of a pass, tilts the gun downward, or overlaps too heavily near the edge. Gravity then pulls the wet film before solvent has flashed, creating sags, curtains, or heavy tear-shaped runs.
The first step is diagnosis. Check whether the defect appears only at the bottom edge or across the full panel. Bottom-only sagging usually comes from poor gun control, excessive fluid, or incorrect pass termination. Full-panel sagging may indicate reducer selection, low booth temperature, poor flash-off, or a coating mixed too thin. Before repair, measure the wetness visually under strong reflection and compare it to a spray-out panel.
Set the gun conservatively. Reduce fluid output slightly, keep the fan stable, and make sure atomization is even. LVLP Spray Gun Deep Penetration, Smooth Blending is valuable when the repair requires controlled film build with less bounce-back, especially on lower rocker areas and vertical bumper profiles. Do not compensate for poor hiding by slowing down at the bottom. Use one additional light pass rather than one heavy wet pass.
The technique is to spray past the edge, not stop on it. Start the trigger before entering the panel and release after leaving the panel. Keep the gun parallel from top to bottom. When using an air spray gun, avoid tilting the nozzle toward the lower edge because the fan will concentrate material into a narrow wet band. Maintain steady travel speed and use 50% to 70% overlap depending on the coating system.
If a run develops during application, do not wipe it. Allow the coating to cure according to the product system. For minor sags in clearcoat, block sand with a fine run-removal block, refine with progressively finer abrasive, then polish after full cure. For basecoat sagging, sanding and reapplication are usually required because metallic or pearl orientation will be disturbed. LVLP Spray Gun Deep Penetration, Smooth Blending helps during the respray by allowing softer edge control and more predictable transfer.
To prevent recurrence, divide vertical parts into upper, middle, and lower zones. Apply the lower zone with slightly faster travel speed and lighter trigger discipline. Respect flash time before clearcoat, and do not bake a wet sag into the finish. Professional results come from consistent material volume, correct pass timing, and a clean exit beyond the lower panel line.
During training, have technicians practice on a vertical scrap panel with tape lines marking pass start, pass exit, and lower edge control. This makes it obvious when the hand slows down near the bottom. In production, use the same discipline: watch the reflection, not the color only. If the lower edge starts looking wetter than the center, adjust the next pass immediately instead of waiting for the sag to form.
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