Also known as: Rust bloom, Rust rash, Rust spotting.
What it looks like
Flash rust is a light, even orange haze that forms on freshly blast-cleaned or water-washed steel. Rust spotting is small rust spots appearing through a finished coating, often in a pattern.
Common causes
- Leaving blast-cleaned steel too long before priming, especially in humid conditions
- Condensation, when the steel temperature is close to the dew point
- Soluble salts on the surface accelerating corrosion
- Water jetting or washing without a suitable inhibitor or a quick prime
- For spotting through the coating: profile peaks poking through a thin film, or areas below the minimum film thickness
Prevention
- Prime blast-cleaned steel as soon as practical, within the time set by the specification
- Keep the steel at least 3 °C above the dew point; use our dew point calculator
- Test for soluble salts where corrosion or coastal exposure suggests them
- Match profile and first-coat thickness so the peaks are covered
Repair
Flash rust on prepared steel usually means re-blasting to the specified grade. Some products and specifications accept light flash rust after water jetting; check before painting over it. Rust spotting in a finished coating needs local preparation and repair to the full system.
Inspector’s note
Flash rust is a timing and climate problem. Logging conditions throughout the shift makes it easy to see when and why it happened.