Common problems during polyurethane application and their solutions

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1. Causes of uneven coating color and failure to cure: a) Phase separation and precipitation of pigments, as well as the lack of mixing or circulation of the material before spraying; b) Blockages in the spray gun’s mixing chamber or filters (especially in polyol components that contain pigments and additives); c) Complete or partial blockage of the isocyanate component.
2. Bubbling: It is important to determine the type of bubbling, its causes, and how to deal with the bubbles that appear between the coating and the substrate. Examining the condition on the back side of the bubble may help identify the cause and solution. For smooth substrates, unclean surfaces (such as oil or water) or moisture/closed gases can be the cause; clean the substrate and apply a primer. For rough or hairy surfaces, moisture can participate in the reaction, as well as an imbalance in the ratio of Component A to Component R, with too much of Component A leading to dryness of the substrate; apply a primer and ensure that the compressed air is dry. Check the operating temperature and pressure, use appropriate mixing chambers and nozzles, and clean the mixing chambers and filters (spray gun, main unit). For viscous or non-curing coatings, incomplete curing of the primer, or an imbalance in the ratio of Component A to Component R, with too much of Component B, can be the cause; stop spraying until the primer has fully cured. Check the operating temperature and pressure, use appropriate mixing chambers and nozzles, and clean the mixing chambers and filters (spray gun, main unit). For bubbling between coats, unclean surfaces of the previous coat (such as oil or water) can be the cause; clean the affected area and polish it. Rough or hairy surfaces, moisture participation, an imbalance in the ratio of Component A to Component R, and too much of Component B can also be causes; ensure that the underlying layer is dry and free from drips or moisture. Ensure that the compressed air is dry. Check the operating temperature and pressure, use appropriate mixing chambers and nozzles, and clean the mixing chambers and filters (spray gun, main unit). For viscous or non-curing coatings, an imbalance in the ratio of Component A to Component R, and too much of Component B can be the cause; check the operating temperature and pressure, and use appropriate mixing chambers and nozzles to clean the mixing chambers and filters (spray gun, main unit).
3. Causes of pinholes: Air or moisture gets trapped in the substrate where coating is applied. During application, the heat generated by the reaction causes the moisture or air to expand, and before the coating cures, the gas pushes through the coating, creating small pinholes in it. Solutions: A. Apply a primer to seal the capillaries. B. Reduce the application temperature; lowering it to 60°C helps to minimize gas expansion. C. Ensure the substrate is fully dried to reduce the amount of gas trapped within it. D. Spray quickly and in thin layers, applying multiple coats—one layer at a time after it has dried—this helps to reduce heat release and thus gas expansion. E. Repair: Quickly fill the pinholes with unhardened polyurethane using your hand. Causes of overspraying: The spray gun isn’t adjusted properly, or the distance between the spray gun and the substrate is too large during spraying. Solution: 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 meters. 5. Delamination of the polyurethane coating from the substrate usually occurs in areas where the substrate has not been properly prepared or where it is contaminated. 6. Separation between layers: This can happen as a result of attempts to repair or maintain the already applied coating surface, or due to an imbalance in the ratio of the two components used. 7. Shrinkage cracks and “fish eyes”: Shrinkage cracks occur due to issues with the materials used during application, or because there is oil or silicone on the substrate surface. “Fish eyes” appear when the substrate surface is too wet, or when there is oil contamination from compressed air on the substrate. 8. Shadow effect: This phenomenon usually occurs when the substrate is not clean or when there is excessive spraying. When applying the coating in one direction, the rapidly curing coating solidifies quickly on top of particles formed by debris or excess spray, failing to completely cover them; as a result, blank spots appear in the coating around those particles. Solution: A should try to clean the surface of the substrate as thoroughly as possible, cleaning it completely before spraying. B Use a suitable spray gun and master the correct spraying technique to minimize overspraying. C Spray in a crisscross pattern across multiple passes to reduce the impact of shadows. D After each spraying pass, promptly check and remove debris and impurities