# Checking

Source: https://coatmentor.com/defects/checking/ · Updated 2026-10-14

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

*Figure: 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](https://coatmentor.com/defects/cracking/):** Cracking goes through at least one coat; checking stays in the top layer.
- **[Alligatoring (crocodiling)](https://coatmentor.com/defects/alligatoring/):** Alligatoring splits the top coat into islands with wide, deep cracks.
- **[Mud cracking](https://coatmentor.com/defects/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).

## Related

- **Concepts:** [Protective coating types](https://coatmentor.com/learn/coating-types/)
- **Standards:** [ISO 4628](https://coatmentor.com/standards/iso-4628/)

## 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](https://coatmentor.com/method/). CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.