Electrophoretic coating is known for its excellent corrosion resistance and uniform coverage, especially on complex metal parts. However, even small defects can significantly weaken the final finish, increase rework costs, and compromise long-term durability. Most coating problems can be traced back to three root causes: poor surface preparation, contamination in the coating bath, and incorrect process parameters such as voltage or temperature. Understanding these defects and their causes is essential for maintaining stable production quality.
1. Pinholing / Outgassing
One of the most common electrocoating defects is pinholing, which appears as tiny holes scattered across the cured film. This defect is caused by air or gas escaping from the substrate during deposition or baking. Porous metals, cast parts, or surfaces with trapped moisture are especially vulnerable. High voltage in the electrophoretic coating process can intensify gassing and prevent the coating film from forming smoothly. Pinholing weakens corrosion resistance and often indicates issues with substrate quality or pretreatment.
2. Orange Peel / Surface Roughness
Orange peel is another frequent electrophoretic coating problem that affects both appearance and coating uniformity. This defect creates a bumpy, uneven texture similar to citrus skin. It usually results from excessive voltage, low solvent content, contamination on the surface, or rough substrate conditions. When the coating cannot level properly during flow-out, the result is a textured finish instead of a smooth, high-quality film. Orange peel suggests that the electrophoretic coating parameters or substrate conditions need adjustment.
3. Cratering / Fish Eyes
Cratering—also known as fish eyes—is characterized by small, round depressions in the coating surface. These electrocoating defects occur when contaminants repel the coating as it flows, preventing full film formation. Common sources of contamination include oil, silicone, dust, and residues from compressed air or handling. Because even microscopic oil droplets can cause visible crater defects, this problem often signals contamination in the electrophoretic coating bath, pretreatment stages, or the operating environment.
4. Poor Adhesion
Poor adhesion is a significant electrophoretic coating problem that results in peeling, blistering, or flaking. The primary cause is insufficient surface preparation. When rust, dirt, oil, or welding residue remains on the surface, the electrophoretic coating cannot bond properly to the metal. Inadequate phosphating or poor rinsing can further weaken adhesion. This defect not only affects appearance but also leads to complete coating failure once the part is exposed to moisture or mechanical stress.
5. Rupturing
Rupturing is the tearing or bursting of the electrophoretic coating film during or after deposition. This defect typically occurs when voltage or temperature is too high, causing rapid deposition and excessive gassing under the film. If the buildup happens too quickly, trapped gas pushes outward and breaks the cured coating. Rupturing indicates unstable electrophoretic coating parameters and should be corrected immediately to prevent production loss and rework.
6. Streaking / Dirt Marks
Streaking appears as vertical lines, dirt trails, or uneven flow patterns on the coated surface. This is one of the most visible electrophoretic coating defects and is commonly linked to contamination, poor rinsing, or filtration issues. Particles dragged into the electrophoretic coating bath can settle on the surface and disrupt film formation. Inconsistent circulation or insufficient bath agitation can also create streak marks. Because streaking directly impacts the final appearance, it is critical to maintain clean tanks and stable bath conditions.
7. Thickness Variation
Thickness variation is a common electrophoretic coating problem, especially in production lines handling parts of different sizes or complex shapes. When the electrical field is unstable—due to poor contact, fluctuating voltage, or parts touching each other—the coating cannot accumulate evenly. This leads to thin areas with weak corrosion protection and thick areas that may trap gas or cure improperly. Small fasteners are particularly prone to sticking together and causing uneven coating build-up.
