Coating defect

Pinholes

Tiny holes through the film, often in groups, that can reach the substrate.

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

Causes

Air or solvent escaping from a porous surface such as inorganic zinc or thermal spray (and concrete for CIP 2).
A full wet coat over a porous primer without a mist coat.
Wrong thinner or spray technique; very hot steel.

Prevention

Apply a mist coat over porous primers, then the full coat.
Use the specified thinner and keep the steel within the temperature limits.

How it is assessed

Visual inspection with a light at a low angle; holiday testing where the specification asks for it.

Telling it apart

Outgassing: Outgassing pinholes gather over porous areas and form after application. Pinholes from spray technique are spread more evenly.
Cratering (fish eyes): Craters are wider dimples with a rim, from contamination.
Holidays and misses: A pinhole that reaches the substrate is a holiday, and the holiday test finds it.

Repair

Abrade and apply another coat, or fill the pinholes as the specification and data sheet say; re-test linings. The specification and the coating manufacturer have the final word on the repair procedure.

29 practice items in the members area mention pinholes (CIP 1: 9 · CIP 2: 9 · FROSIO written: 11).

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.