Coating defect

Alligatoring (crocodiling)

Wide, deep cracks that divide the top coat into islands, like the hide of an alligator.

Alligatoring (crocodiling): the surface as you see it, and a schematic cross-section.

Causes

A hard, rigid coat over a softer, more flexible coat.
A top coat applied before the coat below has dried.
Ageing of oil-based or bituminous coatings under temperature cycling.

Prevention

Use compatible coats with the flexibility in the right order.
Respect the drying and recoat times.

How it is assessed

Visual. Note whether the cracks stop at the top coat or reach the substrate; cracking ratings follow ISO 4628-4.

Telling it apart

Checking: Checking is fine, shallow breaks in the top layer. Alligatoring is wide and deep, and splits the coat into separate islands.
Mud cracking: Mud-cracking is a network of cracks in a coat applied far too thick, typical of zinc silicate. Alligatoring comes from a hard coat over a softer one, or from ageing.
Cracking: Cracking is the general term: rate its density, size and depth under ISO 4628-4.

Repair

Remove the affected coats, usually to the substrate, and apply a compatible system. The specification and the coating manufacturer have the final word on the repair procedure.

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.

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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.