In practical powder coating operations, issues such as paint film discoloration, particle lumps, pinholes, and poor leveling are considered "high-frequency pain points." These not only lower the appearance and texture of the workpiece but also directly affect the mechanical properties and service life of the coating, leading to a surge in rework rates. Many practitioners panic when encountering these problems. In fact, the causes of these problems mostly lie in the core aspects of pretreatment, process parameters, powder quality, and equipment condition. Finding the root cause allows for precise solutions. Today, we will break down 10 common coating problems, clearly outlining their causes and corresponding solutions to help you easily avoid coating pitfalls and improve product pass rates.
1. Paint Film Discoloration
Paint film discoloration is the most obvious appearance defect, such as yellowing, darkening, or the appearance of mixed colors, directly affecting the product's aesthetics. The underlying causes fall into four main categories: First, residual treatment liquid on the workpiece surface, which is not thoroughly cleaned before entering the coating stage; second, improper baking parameters, such as over-baking or the presence of volatile gases in the oven, contaminating the coating; third, accidental mixing of powders of other colors during construction, causing cross-contamination; and fourth, quality issues with the powder itself, such as pigments not being heat-resistant or resins easily yellowing.
The corresponding solutions are clear: First, optimize the pretreatment process to ensure no treatment liquid residue remains on the workpiece surface; second, strictly establish reasonable baking conditions, regularly clean the baking oven, and purify the oven environment; carefully clean the powder spraying system and site before construction to avoid mixing of different colored powders; if the problem is with the powder quality, simply replace it with qualified powder coating.
2. Particles and bumps on the coating surface
Raised particles and bumps on the coating surface are one of the main problems leading to rework. They feel rough and gritty to the touch, severely affecting the smoothness of the appearance. The causes can be investigated from five aspects: 1. Unclean workpiece surface with residual dust and impurities; 2. Uneven powder supply resulting in poor atomization; 3. Powder particles that are too fine, or improper storage leading to moisture and clumping, causing powder accumulation in the spray gun; 4. Unclean painting environment with powder dripping from the spray booth; 5. Poor quality of recycled powder or new powder containing impurities.
Solution approach: Start with basic cleaning to improve the quality of the workpiece substrate, strengthen pretreatment, and ensure surface cleanliness; then adjust process parameters, optimize air pressure, check the spray gun guide, and adjust to the optimal atomization state; strictly control powder particle size distribution, properly store powder, and take moisture-proof measures; regularly clean the painting area and purify the air; recycled powder must be sieved before mixing with new powder, and if the powder quality is too poor, replace it with qualified products.
3. Pinholes
Pinholes are small, round depressions on the coating surface, resembling pockmarks and affecting aesthetics. The core cause is "contamination." Specifically, this includes: residual oil stains on the workpiece surface, incomplete degreasing; compressed air containing water, oil, and other impurities, contaminating the powder; dust or other impurities contaminating the coating environment; and contamination issues inherent in the powder coating itself.
The key to solving these problems is "contamination control and enhanced cleaning": Strengthen the degreasing process during pretreatment to ensure the workpiece surface is free of oil stains; enhance compressed air purification before coating, installing oil and water removal devices; thoroughly clean the coating environment and replace contaminated powder; and implement dust and oil protection throughout the process to avoid secondary contamination.
4. Poor Leveling/Severe Orange Peel Texture
Poor leveling or severe orange peel texture manifests as an uneven coating surface with an orange peel-like texture. The causes are relatively complex: a rough workpiece surface, poor substrate conditions; coarse powder particles, moisture absorption, or poor powder leveling properties; excessively high or low curing temperature affecting powder melting and leveling; uneven coating thickness, too thin or too thick; poor workpiece grounding affecting powder adsorption uniformity; and mixing of different types or manufacturers of powder.
Corresponding solutions: First, improve the surface finish of the workpiece to lay a good foundation for coating; select high-quality powder with uniform particle size distribution and good leveling properties, and store it properly to prevent moisture absorption; precisely adjust curing conditions to ensure suitable temperature; control the coating thickness within a reasonable range of 60-80μm; regularly clean the spraying fixtures to ensure good grounding of the workpiece; thoroughly clean the powder spraying system and site before changing powders, and strictly prohibit the mixing of different powders.
5. Loss of Gloss on the Coating Surface
The coating lacks the expected gloss after drying, appearing dull and gloomy. The main causes are sixfold: poor substrate pretreatment quality, leaving residual impurities on the surface; excessively long baking time or excessively high temperature, leading to coating aging; volatile gases in the oven contaminating the coating; excessive use of recycled powder, affecting gloss; mixing of two powders with different properties, resulting in uneven gloss; and poor quality of the powder coating itself, resulting in insufficient gloss stability.
Solutions: 1. Select a suitable pretreatment process to ensure a clean workpiece surface; adjust appropriate curing parameters to avoid over-baking; clean the oven regularly and purify the gases inside; recycled powder must be filtered before mixing with new powder, with the mixing ratio controlled within 30%; thoroughly clean the powder spraying system when changing powder to avoid powder mixing; if the problem is with the powder quality, directly replace it with high-quality powder.
6. Coating with Pores and Pinholes
Pores and pinholes are penetrating defects in the coating, which reduce the protective performance of the coating and make the substrate susceptible to corrosion. Causes include: pores on the workpiece surface itself; incomplete pretreatment, leaving residual treatment liquid on the workpiece surface, inadequate degreasing and rust prevention; conductive putty applied too thickly and not fully dried before coating; spray gun too close to the workpiece or excessively high application voltage, causing coating penetration; volatile gases contaminating the coating in the drying tunnel; coating too thick or not fully cured; quality problems with the powder coating itself.
Solution: First, strengthen workpiece processing to reduce surface porosity; optimize pretreatment processes to ensure thorough degreasing and rust prevention, with no residual treatment liquid; if using conductive putty, extend the baking time to ensure complete drying; adjust process parameters, controlling the distance between the spray gun and the workpiece and the application voltage; regularly clean the drying tunnel and purify the oven gas; control the coating thickness at 50-80μm and adjust curing conditions to ensure full curing; replace powder that does not meet quality standards promptly.
7. Poor Coating Mechanical Properties
If the coating peels off in large pieces, it indicates severely substandard mechanical properties, rendering it ineffective in its core protective and decorative functions. The main causes are threefold: poor substrate quality and improper pretreatment methods leading to poor adhesion between the coating and the substrate; improper baking conditions, either incomplete curing or over-baking causing coating aging; and poor powder quality or powder that has exceeded its shelf life, resulting in performance failure.
Solutions: 1. Improve pretreatment processes to enhance coating adhesion to the substrate; determine appropriate baking processes based on powder characteristics to avoid insufficient or excessive curing; select qualified powder coatings, and for powders past their shelf life, conduct comprehensive performance testing before use, discarding those that fail to meet standards.
8. Poor Coating Corrosion Resistance
Poor coating corrosion resistance means the coating cannot resist external environmental erosion, leading to substrate rust and damage. Common causes include: insufficient coating curing, resulting in incomplete protective performance; uneven oven temperature with excessive temperature differences, causing some areas of the coating to not cure properly; improper pretreatment of the workpiece, leaving residual treatment solution on the surface; and poor corrosion resistance of the powder coating itself.
Solutions: 1. Precisely adjust baking conditions according to the powder coating's performance to ensure full coating curing; select an oven with good temperature uniformity, ensuring the temperature difference within the oven is controlled within ±5℃; strengthen pretreatment process control to prevent treatment solution residue on the workpiece surface; if the problem is with the powder quality, replace it with a powder coating that meets corrosion resistance standards.
9. Poor Powder Adsorption and Flying
During spraying, powder flies excessively and is difficult to adsorb onto the workpiece surface, resulting in material waste and reduced spraying efficiency. The main causes are sixfold: insufficient or no high voltage generated by the electrostatic generator, preventing effective electrostatic adsorption; poor workpiece grounding, affecting adsorption; excessive spraying air pressure, causing powder to be blown away; blocked air ducts in the recovery device, resulting in insufficient negative pressure; incomplete pretreatment of the workpiece, leaving rust and oil residue on the surface, affecting adsorption; and poor adsorption performance of the powder coating itself.
Corresponding solutions: Inspect the electrostatic generator, repair or adjust the voltage promptly to ensure normal high voltage; regularly clean the spray fixtures to ensure good workpiece grounding; appropriately reduce the spraying air pressure to prevent powder from being blown away; clear blockages in the recovery device to ensure unobstructed air ducts; strengthen the pretreatment process to thoroughly remove rust and oil from the workpiece surface; replace with powder coating with one that meets adsorption performance standards.
10. Uneven Powder Supply
Instantaneous powder supply, fluctuating between high and low, leads to uneven coating thickness, affecting the stability of coating quality. The main causes are concentrated in the powder supply system and compressed air: partial blockage of the powder pipe, resulting in poor powder flowability; insufficient or unstable air pressure; oil and water mixed in the air, contaminating the powder and affecting powder supply; unstable fluidization in the powder feeder, or insufficient powder in the powder feeder.
Solutions: Regularly clean debris or clumps from the powder pipe to keep it unobstructed, and adjust the dry powder flowability of the powder coating; select an air compressor with sufficient and stable pressure to ensure a stable air supply; regularly remove oil and water from the air compressor and install a purification device; maintain stable fluidized bed air pressure, and regularly check and add new powder to ensure sufficient powder in the powder feeder.
11. Summary
In fact, all kinds of problems in powder coating application revolve around five key aspects: pretreatment, powder quality, process parameters, equipment status, and environmental cleanliness. To prevent problems at their root, remember these key points: First, ensure proper pre-treatment to guarantee a clean, residue-free workpiece surface, laying a solid foundation for coating adhesion. Second, use qualified powder coatings, store them properly to avoid moisture and expiration, and avoid mixing powders of different types or manufacturers. Third, precisely control process parameters, especially baking temperature, time, spraying air pressure, and voltage, adjusting them flexibly according to powder characteristics and workpiece conditions. Fourth, perform routine equipment maintenance, regularly cleaning fixtures, powder spraying systems, drying tunnels, and recovery devices to ensure stable equipment operation. Fifth, maintain a clean coating environment, implementing measures to prevent dust, oil contamination, and cross-contamination.
When encountering problems during coating, avoid blindly reworking. First, check the causes listed above one by one to find the root cause and then address it specifically. By implementing meticulous control throughout the entire process, coating defects can be significantly reduced, product quality stability improved, and powder coating made both efficient and worry-free.
