Coating defect

Pinpoint rusting

Many tiny rust spots spread over the surface, often soon after coating.

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

Causes

Profile peaks standing above the film or covered too thinly: the profile was too high for the DFT.
Film too thin overall.
Flash rust or contamination on the peaks before priming.

Prevention

Keep the profile within the specified maximum.
Make sure the first coat covers the peaks; measure DFT over the profile correctly.

How it is assessed

Record the distribution as pinpoint rusting (ASTM D610 type P) and rate the area; check profile and DFT on nearby areas.

Telling it apart

Rust-through (rusting): General rust-through is the end of the coating's life over larger areas. Pinpoint rusting is many tiny spots, often soon after coating.
Contamination and inclusions: Steel grit caught in the film also rusts as small spots, but it shows as lumps you can feel.

Repair

Usually abrade or reblast, then recoat to the correct DFT; the extent sets the method. The specification and the coating manufacturer have the final word on the repair procedure.

5 practice items in the members area mention pinpoint rusting (CIP 2: 3 · FROSIO written: 2).

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.