# Flash rusting

Source: https://coatmentor.com/defects/flash-rusting/ · Updated 2026-10-14

A thin yellow-brown to red-brown film of rust that forms on bare steel within minutes to hours after waterjetting or wet blasting.

*Figure: Flash rusting: the surface as you see it, and a schematic cross-section.*

## Causes

- Wet-cleaned steel drying in air.
- Salt left on the surface or high humidity makes it form faster and heavier.

## Prevention

- Coat soon after cleaning, within the time the specification allows.
- Use an inhibitor only when the coating maker approves it.

## How it is assessed

Grade it light, moderate or heavy against ISO 8501-4 or SSPC-VIS 4 and VIS 5, with a cloth wipe test; the specification sets the grade allowed.

## Telling it apart

- **[Rust-through (rusting)](https://coatmentor.com/defects/rust-through/):** Rust-through comes through a coating in service. Flash rust forms on bare steel before coating, within minutes or hours of wet cleaning.
- **[Pinpoint rusting](https://coatmentor.com/defects/pinpoint-rusting/):** Pinpoint rusting shows through a new coating. Flash rust is on the steel, under where the coating will go.

## Repair

If it exceeds the allowed grade, clean again: waterjet or blast the area. The specification and the coating manufacturer have the final word on the repair procedure.

**26** practice items in the members area mention flash rusting (CIP 1: 9 · CIP 2: 11 · FROSIO written: 6).

## Related

- **Concepts:** [Waterjetting and flash rust](https://coatmentor.com/learn/waterjetting/) · [Surface cleanliness grades](https://coatmentor.com/learn/surface-cleanliness/)
- **Standards:** [ISO 8501](https://coatmentor.com/standards/iso-8501/) · [SSPC-SP, NACE No. and WJ](https://coatmentor.com/standards/sspc-surface-preparation/)

## About this page

Written from coating practice and the published rating standards, in our own words; the drawing is schematic. Checked on 2026-10-14; errors go in the [corrections log](https://coatmentor.com/method/). CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.