Q: What are the main reasons for poor adhesion during recoating?
A: The root cause lies in the "contamination" or "sealing" of the old coating surface, preventing the new and old paint films from effectively bonding.
Surface "isolation film": After long-term exposure, the surface of the pre-coated coil accumulates oil, dust, industrial contaminants, and even powdery residue from aging coatings. Additionally, rust-preventive oil or passivation layers present at the factory can also hinder adhesion.
Overly smooth surface: The smooth surface of the old coating lacks the microscopic roughness that allows the new paint to "grip," naturally resulting in significantly reduced adhesion.
Coating aging: After long-term exposure to sun and rain, the strength and chemical structure of the old coating have changed. If not treated during recoating, delamination is likely to occur.
Q: What steps can be taken to improve adhesion before application?
A: Before application, the surface must be thoroughly cleaned and roughened to establish a reliable bonding interface.
Deep Cleaning: First, use a high-pressure water gun with a neutral detergent to remove dust, oil, and other contaminants from the surface, ensuring a clean finish.
Surface Sanding: After cleaning and drying, use sandpaper or a sandblasting machine to evenly sand the old coating to remove the aged layer and create a new, roughened surface.
Chemical Treatment: After sanding, a phosphating solution or silane treatment agent can be used for chemical treatment. This forms a conversion film on the surface that can chemically bond with both the old and new coatings, further improving adhesion.
Q: What are some tips for choosing the right recoating paint?
A: Choosing the right paint is fundamental to a successful recoating, and it's essential to ensure chemical compatibility between the old and new coatings.
**Use dedicated, matching paints:** Choose paints specifically designed for galvanized parts, such as epoxy zinc yellow primer or phosphate primer. These paints contain special functional groups that form a strong chemical bond with both the galvanized layer and the old coating.
**Matching paint properties:**
**Strong adhesion:** The paint itself must possess excellent adhesion.
**Good weather resistance:** Ensure the recoated layer can withstand long-term exposure to wind and sun.
**Low surface tension:** The surface tension of the chosen repair paint should be lower than that of the old coating to allow for thorough wetting, penetration, and a strong bond.
**Special formulation:** High-quality repair paints often contain solvents that swell the old coating or polar groups that form hydrogen bonds, thereby improving interlayer adhesion.
Q: How is adhesion tested and judged?
A: The most standard method is the "cross-cut test," which quantitatively assesses adhesion level.
Test Method: Use a cross-cut tester to draw a grid pattern (usually 25 squares) on the coating surface. Then, apply special tape and quickly tear it off. The grade is determined by the area of coating peeling off within each square.
Grading Standard: The industry commonly uses a 0-5 or 0B-5B grading standard, where 0 or 5B is the best (no peeling), and 5 or 0B is the worst (severe peeling).
Grade Requirements: For applications such as recoating, the industry typically requires an adhesion level of at least 3B or, more stringently, 0, to ensure application quality.
Q: What are the environmental requirements for application?
A: A suitable environment is essential for proper curing of the coating. Excessive temperature, low temperature, or high humidity can all lead to failure.
Temperature: The recommended ambient temperature for application is between 5℃ and 35℃.
Humidity: The relative humidity should be below 75%. Avoid application in rainy or foggy conditions.
Application Techniques: When recoating, use the principle of "thin coats, multiple coats," allowing sufficient drying time between each coat to prevent cracking or incomplete drying due to excessively thick coatings.