1. During pickling, the acid at the crack is not neutralized or incompletely neutralized, the phosphating treatment is not complete, sheet metal fabrication， and the film is not completely rusted. After painting, rust can spread and spill over cracks over time.

2. After strong acid pickling and strong alkali degreasing treatment, the chemical substances remaining in the cracks are neutralized with each other, and then do not wash with water, clean and dry. After the cracks have dried, apply a white product for another period of time. These white products are produced by corrosive substances in the air. The problem mostly occurs in cracks where chemical reactions spill over between March and November.

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