Long, thread-like marks in the wet film are often described as stringing, tails, or streaking. They can appear as narrow lines at the top or bottom of the fan, a central material rope, or repeated directional ridges across a panel. The defect should not be corrected by random pressure changes. A professional diagnosis separates coating rheology, nozzle wear, air-cap restriction, and fluid-to-air imbalance.
1. Capture the defect on test paper
Stop spraying the vehicle and reproduce the problem on masking paper. Use the same coating, temperature, gun distance, fluid setting, and dynamic inlet pressure. Make a one-second static pattern followed by two normal-speed passes. Record whether the defect appears at the fan edge, through the center, only at trigger start, or continuously.
Edge tails usually indicate poor atomization at the fan horns, a blocked air-cap port, or excessive material for the available atomizing air. A central rope can indicate high viscosity, a worn or oversized fluid nozzle, or insufficient needle lift control. Intermittent strings may come from dried coating breaking loose inside the nozzle.
2. Separate air-cap and fluid-side faults
Clean the air-cap face, horn holes, center bore, and auxiliary passages with approved tools. Rotate the cap 180 degrees and repeat the static pattern. If the defect rotates with the cap, the problem is in the cap or its air supply. If it remains in the same position, inspect the needle, fluid nozzle, coating condition, and gun-body fluid path.
Do not enlarge a port with wire or a drill. Calibrated hole size controls fan balance, and even a small change can permanently distort atomization.
3. Inspect needle and nozzle wear
Depressurize the gun, empty the cup, remove the needle first, and then loosen the nozzle. Under magnification, inspect the nozzle orifice for an oval shape, burrs, impact marks, cured coating, or asymmetric polishing. Roll the needle on a flat glass plate and inspect its tip for bending, flats, or an uneven sealing ring.
A lvlp spray gun Professional Automotive Tools service standard should treat the needle and fluid nozzle as a matched set. Combining a new needle with a worn nozzle may stop leakage temporarily while leaving inconsistent material flow.
4. Verify material condition
Confirm mix ratio, pot life, reducer grade, and material temperature. Measure viscosity with the coating supplier’s specified cup. Cold or partially reacted coating can form filaments because it does not break into droplets cleanly. Strain the material and inspect the filter for skin, gel particles, or pigment agglomerates.
If the coating has exceeded pot life or shows gel formation, discard it according to shop procedure. Do not compensate with extra solvent unless the technical data sheet specifically permits an adjustment.
5. Rebalance fluid and atomizing air
Return the gun to the manufacturer’s baseline dynamic pressure. Open the fan to the validated working width, then reduce fluid delivery until the center is even without becoming starved. Increase pressure only in small increments while the trigger is fully pulled. Observe whether droplets become uniformly fine or whether the fan develops dry edges.
An air spray gun must be evaluated under continuous flow because regulator response, hose restriction, couplers, and compressor recovery can change the available air during a full pass. Static gauge readings are not sufficient.
6. Validate before returning to the panel
Spray a fresh static pattern, a horizontal pattern, and a production-speed overlap sequence. Inspect for symmetry, edge tails, central loading, pulsing, and droplet consistency. For metallic basecoat, also check particle orientation and mottling. Record the successful nozzle set, viscosity, dynamic pressure, fan position, fluid turns, and material temperature.
Final control point: string-like defects are evidence of a process imbalance. Correct the mechanical or material cause before modifying technique.
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