Coating defect

Cracking

Breaks in the film, from fine hairline cracks in the top coat to deep cracks down to the substrate.

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

Causes

Film applied too thick.
A coating too brittle for the movement or temperature changes of the substrate.
Ageing and UV embrittlement.
A hard coat over a soft, still-moving coat.

Prevention

Keep DFT within the maximum, especially at corners and welds.
Use a system suited to the service temperature and substrate movement.

How it is assessed

Rate density, size and depth under ISO 4628-4: does the crack stop in the top coat or reach the substrate?

Telling it apart

Checking: Checking stays within the top layer; cracking goes through at least one coat.
Mud cracking: Mud-cracking is a network of plates in a coat applied far too thick.
Alligatoring (crocodiling): Alligatoring is wide cracks that split the top coat into islands.

Repair

Shallow cracks may be sealed by overcoating if the spec allows. Deep cracks need removal of the cracked coating and recoating. The specification and the coating manufacturer have the final word on the repair procedure.

52 practice items in the members area mention cracking (CIP 1: 16 · CIP 2: 21 · FROSIO written: 15).

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.