Coating defect

Flash rusting

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

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): 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: 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).

Try it in an exam-style question. The free readiness check shows the question style, explains every answer and gives you a dated study plan.

Take the free readiness check See the tracks

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. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.