Coating defect

Checking

Fine, short breaks in the surface of the top coat that do not go through it. Some need a 10× magnifier to see.

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

Causes

Weathering makes the surface brittle.
Film too thick, or a coating with too little flexibility for the service.
Application at too high a temperature.

Prevention

Choose a coating suited to the exposure and apply it within the thickness range.

How it is assessed

Use a 10× magnifier to tell checking from cracking: checking stays within the last coat; cracking goes through at least one coat.

Telling it apart

Cracking: Cracking goes through at least one coat; checking stays in the top layer.
Alligatoring (crocodiling): Alligatoring splits the top coat into islands with wide, deep cracks.
Mud cracking: Mud-cracking runs through a thick primer coat as a network of plates.

Repair

Sand or abrade the checked coat and apply a new coat before the checks grow into cracks. The specification and the coating manufacturer have the final word on the repair procedure.

10 practice items in the members area mention checking (CIP 1: 4 · CIP 2: 4 · 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.