Why do paint spraying often result in shrinkage, thickening, or widening of the edges?

During paint spraying, defects such as shrinkage, thick edges, and runs are prone to occur at corners. This article analyzes in detail the core causes of these problems, including surface tension imbalance, differences in solvent evaporation rates, insufficient substrate surface energy, and the influence of application techniques. It also introduces solutions such as wetting and leveling agents, substrate treatment, spraying techniques, and formula optimization, applicable to automotive paints, industrial paints, wood coatings, and anti-corrosion coatings. In automotive painting, industrial anti-corrosion, wooden furniture, and metal coating processes, corners often exhibit the following defects: shrinkage, thick edges, runs, and paint buildup. These problems not only affect the appearance but also reduce the overall quality and lifespan of the coating. Many construction workers believe this is a problem with the spray gun or the paint itself, but in reality, the vast majority of corner defects are closely related to “surface tension balance” and “edge evaporation rate.” This article will systematically analyze the formation principles of spraying corner defects and common solutions in the industrial coatings industry. I. What are shrinkage, thick edges, and runs? Before formally analyzing the causes, it is necessary to understand these common phenomena. 1. **Edge Shrinkage:** Edge shrinkage refers to the paint noticeably shrinking back in the corner areas, resulting in: thinner paint film, exposed substrate, incomplete coverage, and in severe cases, noticeable edge gaps.
2. **Fat Edges:** Fat edges are typically characterized by a paint film that is significantly thicker than the surrounding surface at the corners. This leads to: uneven gloss, raised edges, and an unbalanced appearance.
3. **Thick Edges:** Thick edges are a further aggravation of fat edges. When too much paint accumulates at the corners, it easily leads to: excessively thick paint film, sagging, slow drying, and an increased risk of localized cracking.
II. **Core Cause of Corner Problems:** Surface Tension Imbalance
After spraying, the paint film is a liquid fluid. Liquids naturally flow from “low surface tension areas” to “high surface tension areas.” This is the key reason for corner problems.
III. **Why Are Corners More Prone to Problems?**
1. **Faster Solvent Evaporation at Corners:** Corners typically have: a larger surface area exposed to air, faster heat dissipation, and a higher solvent evaporation rate. When the solvent evaporates rapidly: the surface tension in the corner area increases rapidly. The solvent evaporates more slowly in the central area, resulting in relatively low surface tension. This creates a “tension difference,” ultimately causing a change in the paint’s flow direction. IV. Why does paint flow towards the edges? According to the laws of liquid flow: liquids typically flow from low-tension areas to high-tension areas. Therefore: the central area has low tension, while the edge area has high tension, causing the paint to gradually move towards the edges. This ultimately results in: edge buildup, thickened paint film, thick edges, and bulky edges. If the tension change is more drastic, it can also lead to: edge shrinkage, undercoat leakage, and backflow. These phenomena are particularly noticeable in high-gloss topcoats. V. Substrate surface energy mismatch is also a significant factor. Besides solvent evaporation, the substrate condition is equally crucial. If the substrate has: oil stains, dust, mold release agent residue, or excessively low surface energy, it will cause: the paint to fail to evenly wet the substrate. This results in: localized backflow, pinholes, edge shrinkage, and poor adhesion. This is especially important for: plastic parts, aluminum, galvanized sheets, and high-gloss substrates, where surface tension matching is even more critical. VI. Manifestations of Edge and Corner Problems in Different Coating Systems 1. Automotive Paint: In automotive painting, edges and corners are prone to: thick edges, runs, and gloss differences, especially noticeable in high-solids automotive clear coat systems. 2. Industrial Anti-corrosion Paint: When painting industrial equipment, thick edges may lead to: uneven drying, increased internal stress in the paint film, and later cracking, particularly common in steel structure edge areas. 3. Wood Coating: Wood edges absorb more moisture. Uneven application can easily result in: dark edges, inconsistent paint film thickness, and sanding penetration. VII. How to Solve Edge Shrinkage, Thick Edges, and Flawed Edges? Truly mature industrial coating systems typically address these issues through: formula optimization, additive balancing, and application process control. VIII. Solution 1: Adding Wetting and Leveling Agents Wetting and leveling agents are commonly used coating additives. Its main functions include: reducing surface tension, improving wetting performance, enhancing leveling effect, and reducing edge buildup. Proper use of leveling agents can effectively reduce: edge shrinkage, pinholes, thick edges, and brush marks. Currently, it is widely used in: automotive paints, industrial paints, UV coatings, and wood coatings. IX. Solution Two: Strengthen Substrate Treatment. Before spraying, ensure the substrate is clean. Standard treatment procedures include: degreasing, dust removal, sanding, and increasing surface energy. If necessary, flame treatment, corona treatment, and plasma treatment are also required, especially for low surface energy materials. X. Solution Three: Optimize Spraying Process. The application process has a significant impact on edge quality. Correct practices include: applying thin, multiple coats to edges and corners, avoiding a single thick coat, controlling the spray gun distance, controlling the spray air pressure, and adjusting the atomization effect. This reduces concentrated liquid buildup at edges. XI. Solution Four: Properly Adjust Paint Viscosity. Appropriately increasing the system viscosity can reduce excessive paint flow. However, higher viscosity is not always better. If the viscosity is too high, it may lead to: poor leveling, orange peel, and haze. Therefore, a comprehensive balance is needed for: flowability, leveling, and edge coverage. XII. How do modern paint formulations solve edge problems? Modern industrial paint manufacturers typically achieve overall stability control through: synergistic balancing of additives, including: wetting agents, leveling agents, defoamers, solvent systems, and resin combinations. High-end systems further optimize: evaporation gradient, surface tension curve, edge wetting ability, and film rheological properties to improve edge consistency. XIII. How to reduce edge rework rate? For spraying companies, edge defects directly increase: sanding rework, material waste, labor costs, and production cycle. Therefore, it is recommended to: establish a standardized spraying system, including: fixed dilution ratio, fixed spray gun parameters, fixed application temperature and humidity, and fixed flash-off time. This can significantly improve stability. Conclusion: Edge shrinkage, thick edges, and large edges during paint spraying are essentially the result of the combined effects of paint surface tension, evaporation rate, and substrate wettability. Whether it’s automotive paint, industrial anti-corrosion paint, or wood paint, achieving a smooth and uniform edge finish requires: a reasonable formula system, precise additive balance, scientific application techniques, and proper substrate preparation. Only through comprehensive optimization can a high-quality spraying effect with uniform paint film edges, stable appearance, and long-term durability be truly achieved.