Airless Spray Application Defect
A contractor is applying a zinc-rich primer using airless spray to a bridge structure. The specified DFT is 75 microns. The contractor uses a 0.017-inch tip, 2000 psi pressure, and a 50:1 ratio pump. The ambient temperature is 30°C, and the coating viscosity is 80 KU. After application, the inspector notices a mottled appearance with uneven thickness, and some areas have a dry spray. The contractor claims the equipment is adequate and blames the coating.
1. Identify potential causes of the mottled appearance and dry spray.
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Potential causes: 1) Incorrect tip size: 0.017-inch tip may be too small for a zinc-rich primer at 80 KU viscosity, leading to insufficient atomization. 2) Pressure too low: 2000 psi may be insufficient for proper atomization; zinc-rich primers often require higher pressure (e.g., 2500-3000 psi). 3) Viscosity too high: 80 KU is high for airless spray; thinning may be needed per manufacturer's recommendations. 4) Temperature: 30°C may cause solvent flash-off too quickly, leading to dry spray. 5) Spray technique: Improper gun distance or speed. 6) Equipment wear: Pump or tip wear could reduce pressure. The mottled appearance indicates uneven film build, possibly due to poor atomization or inconsistent spray pattern.
2. What adjustments should be made to improve the application?
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Adjustments: 1) Increase tip size to 0.019-0.021 inch for higher viscosity material. 2) Increase pressure to 2500-3000 psi, ensuring the pump can maintain pressure. 3) Check viscosity and thin if necessary according to the manufacturer's data sheet. 4) Control environment: If possible, lower ambient temperature or use slower-evaporating solvent. 5) Verify spray technique: Maintain proper gun distance (30-45 cm) and perpendicular strokes. 6) Inspect equipment: Replace worn tips and check filters. 7) Test spray on a sample panel to confirm atomization and film build. The contractor should demonstrate that the adjusted parameters produce a uniform coating meeting the specified DFT.