1.How to remove surface impurities?
Residual oil (rolling oil), ash (zinc oxidation products), dust, or fingerprints on the surface of galvanized coils can cause "false adhesion" (partial failure of the coating to adhere). A multi-stage degreasing and water washing process is required: first, spray or soak with an alkaline degreaser (such as sodium hydroxide solution, 1-3% concentration) to remove oil; then rinse with a high-pressure water jet (0.3-0.5 MPa) to remove any degreaser residue; and finally, rinse with deionized water (conductivity ≤ 50 μS/cm) to prevent impurities in the water (such as calcium and magnesium ions) from forming spots on the surface.
2.How to control the surface state of the zinc layer?
The zinc layer must be uniform (thickness deviation ≤ 5%), free of missing plating and zinc particles (protruding zinc crystals) - zinc particles can cause local uneven thickness of the coating or even rupture. If the zinc layer is oxidized and blackened (due to a humid storage environment), it must first be removed by light shot blasting or chemical pickling (such as dilute hydrochloric acid) to expose the fresh zinc surface (but pre-treatment must be performed immediately after pickling to avoid secondary oxidation).
3.Why should we increase the surface roughness moderately?
A smooth zinc surface is not conducive to coating adhesion, and a "micro-rough surface" can be formed through phosphating or passivation treatment: during phosphating, uniform phosphate crystals (film thickness 1-3μm) are generated on the surface of the zinc layer. The pores between the crystals allow the paint to "embed", significantly improving adhesion; passivation (such as chromate passivation) forms a dense passivation film, which not only enhances the corrosion resistance of the zinc layer, but also combines with the coating through chemical adsorption.
4.How to match paint types by scene?
For outdoor applications (such as building roofs and curtain walls): Choose weather-resistant coatings (such as fluorocarbon coatings and silicone-modified polyester coatings). Fluorocarbon coatings contain fluorine atoms, which have high chemical bonding energy and offer strong resistance to UV aging and acid rain (with an outdoor lifespan of up to 15-20 years). Silicon-modified polyester coatings offer greater cost-effectiveness (with a lifespan of 10-15 years). Ensure that the amount of weathering additives (such as UV absorbers) added to the coating meets regulatory requirements (typically 1-3%).
For industrial applications (such as appliance housings and equipment panels): Choose high-adhesion and decorative coatings (such as polyester coatings and epoxy coatings). Epoxy primers offer strong adhesion (cross-cut test ≥ 0), paired with polyester topcoats (gloss 50-80%), for both corrosion protection and aesthetics. For applications involving oily surfaces (such as kitchen appliances), choose oil-resistant coatings (such as polyurethane coatings). Special scenarios (such as chemical plants and coastal areas): Choose acid- and alkali-resistant/salt spray coatings (such as vinyl coatings and polyurea coatings) - Vinyl coatings are highly resistant to chemicals and can withstand acid and alkaline environments with a pH of 2-12; after a salt spray test (500 hours), the coating will not blister or rust.
5.How to keep the coating stable?
Before use, the paint must be thoroughly stirred (to avoid pigment settling) and the viscosity must be controlled (test with a 4-cup coating cup, 20-30 seconds, adjust according to the coating method: slightly higher viscosity for roller coating, slightly lower for spraying). Excessively high viscosity will result in a thick and uneven coating, while too low viscosity will cause sagging (the coating will be too thin in some areas). If the paint has been stored for too long (exceeding its shelf life), its fineness (≤20μm) and solid content (≥50%) must be tested; if unqualified, it is prohibited from use.