1.Why does direct spray paint have poor adhesion?
Too smooth a surface: The zinc coating has dense crystals and low surface energy, making it difficult for paint to "grab."
Chemical inertness: The zinc coating often has a thin oxide or passivation film on its surface. This film prevents further corrosion, but also prevents effective chemical bonding between the paint and the zinc.
If you forcefully spray paint directly, the paint film may appear fine in the short term, but after a period of thermal expansion and contraction, mechanical vibration, or humidity, it will begin to flake off.
2.What are some ways to increase roughness?
The best method: Phosphating. This chemical process creates a crystalline phosphate film on the zinc surface. This film is porous and strongly bonded to the substrate, making it an ideal base for paint. This method is commonly used in industrial continuous production.
Common methods: Sanding or polishing. Use a very fine abrasive (such as 120-grit sandpaper or higher) for light sanding or hand-polishing to dull the surface and slightly roughen it. Important: Use gentle pressure to only polish away the passivation film; do not wear through the zinc layer.
Simple method: Natural weathering: Leave the galvanized coil outdoors for 3-6 months to allow rain and air to naturally form a layer of zinc salts, which aids adhesion. However, this method takes too long and the results are inconsistent, so it is not recommended.
3.How to choose the right primer?
Epoxy zinc yellow primer: This is a traditionally preferred choice specifically designed for galvanized surfaces, offering excellent adhesion.
Epoxy zinc-rich primer: Offers excellent compatibility and rust protection.
Water-based acrylic primer: Environmentally friendly and a current trend.
Avoid: Alkyd and oil-based primers. The oils in these paints can saponify the zinc, causing the paint film to flake.
4.How to spray paint color coated coils?
On the color-coated plate production line, the galvanized coils undergo a series of automated processes including rigorous cleaning, chemical pretreatment (such as phosphating and passivation), coating with special primers, baking, and topcoating. The paint film quality, durability, and adhesion are far beyond the reach of on-site manual spraying.