Has rust reappeared after a while since painting? The cause may not be the paint film, but rather corrosion of the substrate due to insufficient rust prevention treatment.
Rust that reappears after painting may not be due to the paint film itself, but rather insufficient anti-corrosion treatment. However, on-site, it is often mistakenly judged that "since it has been painted, it won't rust." In reality, if fine red rust or corrosion components remain on the surface of the material, corrosion can progress inside the paint film and manifest as reappearing rust months later. This is especially significant at welded joints and cut surfaces, where the effects of insufficient anti-corrosion treatment are more pronounced. This is actually a point that is often overlooked. Even if it looks clean on the outside, corrosion reactions are progressing inside the paint film. Furthermore, if anti-corrosion primer is not applied after blast treatment and left unattended, reappearing rust can rapidly progress in conditions of over 60 percent humidity. A common misconception on-site is the belief that "if the topcoat is thick enough, there will be no problem." By reviewing the anti-corrosion process, there are cases where reappearing rust can be significantly improved. Actual improvement cases are also detailed in publicly available materials.
