Coating defect

Mud cracking

A network of cracks that splits the film into plates, like dried mud. Typical of inorganic zinc silicate and some water-borne coatings applied too thick.

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

Causes

Film far above the maximum DFT, often at welds, corners and overlaps.
Fast drying of the surface while the film below shrinks.
Inorganic zinc silicate applied well above the maximum DFT that its data sheet gives.

Prevention

Control the wet film with a comb gauge as you apply.
Avoid building up film on details and overlaps.

How it is assessed

Visual inspection, supported by DFT readings in the affected area against the specified maximum.

Telling it apart

Checking: Checking is fine breaks in the surface of a top coat. Mud-cracking splits a thick coat into plates through its full depth.
Alligatoring (crocodiling): Alligatoring is a top coat that cracks over a softer coat below. Mud-cracking is a single coat that was too thick.

Repair

Remove the cracked film, usually by blast cleaning, and reapply within the DFT range. Overcoating does not repair it. The specification and the coating manufacturer have the final word on the repair procedure.

11 practice items in the members area mention mud cracking (CIP 1: 6 · 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.