In the metallic paint application process, sagging and orange peel are common surface defects that directly affect the smoothness and gloss of the paint film. Sagging manifests as paint flowing downwards on a vertical surface, forming teardrop-like bumps, while orange peel, due to insufficient leveling of the paint film, presents an uneven texture similar to orange peel. Avoiding these two defects requires a comprehensive approach encompassing environmental control, material selection, process parameters, and operational techniques.
Ambient temperature and humidity are the primary factors affecting the quality of metallic paint application. The spray painting space must be kept dry and well-ventilated, with the temperature controlled within a suitable range and the humidity within a reasonable range. Excessive temperature causes the paint to dry too quickly, resulting in uneven solvent evaporation and easily causing orange peel; excessively low temperature increases the viscosity of the paint, worsening leveling and increasing the risk of sagging. Excessive humidity allows moisture to mix into the paint film, disrupting surface tension and causing the paint layer to whiten or sagging. Therefore, the environment should be controlled before spray painting using air conditioning or dehumidification equipment, and spraying should be avoided on rainy days or during periods of high temperature. Material selection and formulation are crucial for preventing defects. The viscosity, thinner type, and hardener ratio of metallic paint directly affect the paint's flowability and drying speed. The paint, thinner, and hardener must be accurately weighed according to the recommended ratio in the metallic paint instructions to avoid an imbalance that results in an overly thin or thick paint. For example, adding too much thinner will lower the paint viscosity and increase the tendency for sagging; while excessive hardener may cause the paint film to dry too quickly, leading to orange peel. Furthermore, high-quality thinners and hardeners compatible with metallic paints must be selected; inferior additives may disrupt the paint film formation process due to mismatched evaporation rates or unstable chemical properties.
Precise control of spray gun operating parameters is key to avoiding defects. The distance between the spray gun and the paint surface must be maintained within a reasonable range. Too close a distance will cause paint buildup and sagging; too far a distance will cause the paint mist to disperse, resulting in insufficient paint film thickness and an orange peel effect. The spray gun movement speed must be uniform and steady, avoiding pauses or repeated spraying of the same area, otherwise, it will cause localized excessively thick paint layers. The spraying angle must be kept perpendicular; tilting the spray gun will result in uneven paint film distribution, increasing the risk of sagging and orange peel. Furthermore, the air pressure of the spray gun needs to be adjusted according to the viscosity of the paint. Too high an air pressure will result in excessively fine paint mist particles, leading to excessively rapid drying; too low an air pressure will result in excessively coarse paint mist particles, reducing leveling properties.
Thin-layer spraying is an effective process for preventing sagging and orange peel. Applying too thick a single coat is the direct cause of sagging, while thin-layer spraying reduces this risk through layered drying. Allow adequate drying time after each coat, allowing the paint film surface to partially cure before applying the next coat. For non-horizontal surfaces, shorten the single-layer spraying time or increase the spray gun movement speed to prevent the paint from flowing downwards due to gravity. In addition, using a "half-gun over half-gun" technique, where each spray gun movement covers the edge of the previous coat, ensures uniform paint film coverage and reduces boundary lines and thickness differences.
The pretreatment quality of the metal substrate is crucial to paint film adhesion and leveling. The substrate surface must be thoroughly cleaned to remove oil, dust, and rust; otherwise, the adhesion between the paint and the substrate will be affected, leading to sagging or orange peel. For smooth metal surfaces, grinding or sandblasting is necessary to increase roughness and improve paint film adhesion. After grinding, the surface should be wiped with a cleaning agent to remove residual debris and prevent impurities from mixing into the paint layer. Furthermore, the substrate temperature must be consistent with the ambient temperature during painting; excessive temperature differences will cause uneven paint drying and defects.
The drying and curing stages after painting are equally important. The paint film needs to dry fully in a well-ventilated environment with stable temperature and humidity, avoiding external contact or dust contamination of the uncured film. During drying, direct sunlight or strong winds should be avoided, as these will cause the paint film to dry too quickly, hindering the evaporation of internal solvents and resulting in orange peel or pinholes. For metallic paints, the curing time must strictly follow the instructions; premature subsequent treatments (such as polishing) may damage the incompletely cured paint film, causing sagging or uneven gloss. To avoid runs and orange peel in metallic paint coatings, a systematic prevention and control system is needed. From environmental control to material selection, from spray gun operation to process optimization, every step must be strictly controlled. By precisely controlling parameters, standardizing operating procedures, and strengthening process management, the quality of the paint film can be significantly improved, achieving a smooth, glossy, and high-gloss finish on the metallic surface.
