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, along with the fact that the material was not stirred or circulated before spraying; b) Blockage in the 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: The most important thing when bubbling occurs is to determine its type, the cause, and how to deal with it. For bubbling between the coating and the substrate, examining the condition on the back side of the bubble can help identify the cause and solution. Smooth substrates: Unclean surface (such as oil or water); moisture/enclosed gases – clean the substrate and apply a primer. Rough/rough-textured surfaces: Moisture participates in the reaction; imbalance in the ratio of A to R components, with too much A component, causing the substrate to remain dry – apply a primer and ensure that the compressed air is dry; check the operating temperature and pressure; use an appropriate combination of mixing chamber and nozzle; clean the mixing chamber and filters (gun, main unit). Thick/sticky/coating that does not cure: Incomplete curing of the primer; imbalance in the ratio of A to R components, with too much A component – wait until the primer has fully cured before proceeding; check the operating temperature and pressure; use an appropriate combination of mixing chamber and nozzle; clean the mixing chamber and filters (gun, main unit). Bubbling between coats: Examine the condition on the back side of the bubble to identify the cause and solution. Smooth surfaces: Unclean surface of the previous coat (oil, water) – clean the area and polish it. Rough/rough-textured surfaces: Moisture participates in the reaction; imbalance in the ratio of A to R components, with too much A component; ensure the underlying layer is dry; be careful of droplets or moisture; ensure that the compressed air is dry; check the operating temperature and pressure; use an appropriate combination of mixing chamber and nozzle; clean the mixing chamber and filters (gun, main unit). Thick/sticky/coating that does not cure: Imbalance in the ratio of A to R components, with too much A component; check the operating temperature and pressure; use an appropriate combination of mixing chamber and nozzle; clean the mixing chamber and filters (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) Lower the application temperature to 60°C to reduce the degree of gas expansion; c) Ensure the substrate is fully dry to decrease the amount of gas trapped within it; d) Spray quickly, in thin layers and frequently, applying multiple coats one after another. This helps to reduce heat release, thereby minimizing gas expansion. e Repair: Quickly fill the unfixed polyurethane into the pinholes with your hand. 4. Causes of overspray: The spray gun is not properly adjusted, 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 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. Inter-layer separation can result from attempts to repair or maintain the already applied coating surface, as well as from an imbalance in the ratio of the two components used. 6. Shrinkage cavities and fish eyes: Shrinkage cavities (dents) occur due to issues related to the application process, the quality of the materials used, or oil or silicone residues on the substrate surface. Fish eyes appear when the substrate surface is too wet, or when oil present in the compressed air contaminates the substrate. 7. 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, shadows around these particles create gaps in the coating layer. 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 and master the correct spraying technique to reduce overspray. c Apply paint in a crisscross pattern across both vertical and horizontal directions, using multiple coats to reduce the impact of shadows. d After each coat is applied, check and remove any debris and impurities promptly.