Surface is near area with high humidity such as river, soil or sea. Other possible causes include expired oil paint system, using low-quality paint which composes of cheap pigment, ignorance to use anti-rust primer or using low-quality anti-rust primer.
Appearance Rusts exist and push oil paint film to flake.
Solution Scrape old paint and rust down to original metal surface and prepare with primer within 4 hours. Apply the anti-rust primer once and refinish the surface twice.
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