Coating defect

Rust-through (rusting)

Rust spots, rust staining or general rusting showing through the coating, often first at edges, welds, bolts and damage.

Rust-through (rusting): the surface as you see it, and a schematic cross-section.

Causes

The coating has reached the end of its life.
Film too thin, especially on edges and welds without a stripe coat.
Holidays and damage left unrepaired.
Poor surface preparation under the coating.

Prevention

Stripe coat edges, welds and details.
Check DFT, holiday test where specified, and repair damage.

How it is assessed

Rate the rusted area against reference pictures: ISO 4628-3 (Ri 0 to Ri 5) or ASTM D610 with SSPC-VIS 2. Record whether the rust is spot, general, pinpoint or hybrid.

Telling it apart

Pinpoint rusting: Pinpoint rusting is many tiny spots, often soon after coating, from the profile or a thin film.
Edge corrosion (thin film at edges): Rust that starts only on edges and corners, while the faces are sound, is edge corrosion.
Flash rusting: Flash rust forms on bare steel before coating.

Repair

Spot repair, overcoat or full removal, chosen from the extent of rusting and the condition of the remaining coating. The specification and the coating manufacturer have the final word on the repair procedure.

24 practice items in the members area mention rust-through (CIP 1: 9 · CIP 2: 8 · FROSIO written: 7).

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.