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1. Causes of pinholes: Air or moisture gets trapped on the substrate to be coated. During coating application, the heat generated by the reaction causes the moisture or air to expand; before the coating cures, these gases push their way out through the coating, resulting in small pinholes on the coated surface. Solutions: a) Apply a primer to seal the capillaries; b) Reduce the application temperature to 60°C to minimize gas expansion; c) Ensure the substrate is fully dried to reduce the amount of gas trapped within it; d) Spray quickly, in thin layers and frequently, applying additional coats only after each layer has dried. This helps to reduce heat release and thus limits gas expansion. e Repair: Quickly fill the unhardened polyurethane into the pinholes with your hand. 2. Causes of overspray: The spray gun isn’t adjusted properly, or the distance between the spray gun and the substrate is too large during spraying. Solutions: a Choose a spray gun with an appropriate output volume. b Adjust the distance and angle between the spray gun and the substrate. Under normal conditions, the spray gun should be at a right angle to the substrate, at a distance of 0.5–0.6 m. Separation of layer A of the polyurethane coating from the substrate usually occurs in areas where the substrate has not been properly prepared or is contaminated. Layer separation can also result from attempts to repair or maintain the already applied coating surface. An imbalance in the ratio of the two components can also lead to this issue. 4. Shadow effect: This phenomenon occurs typically when the substrate is not clean or when there is excessive spraying. When applying the coating in one direction, the rapidly curing coating will solidify quickly on top of debris or particles formed by excessive spraying, failing to completely cover them; as a result, shadows around these debris or particles create gaps in the coating. The coating voids caused by the shadow effect should not be confused with pinholes. Solution: a Try to clean the surface of the substrate as thoroughly as possible; clean it completely before spraying. b Use a suitable spray gun, master the correct spraying technique, and reduce overspray. c During spraying, apply coats in a crisscross pattern to minimize the impact of shadows. d After each coating is applied, check and remove debris and impurities promptly.