Coating defect

Chalking

A powdery layer on the surface that wipes off on a finger or cloth, with loss of gloss and colour.

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

Causes

UV light breaks down the binder at the surface, freeing the pigment. Epoxies chalk quickly in sunlight.
A top coat that is not UV-stable for the exposure.

Prevention

Use a UV-stable top coat, such as an aliphatic polyurethane or a polysiloxane, where the coating sees sunlight.
Expect epoxy primers and intermediate coats left in the sun to chalk. Overcoat them within the recoat window, and wash off any chalk first.

How it is assessed

Rate with the tape method of ISO 4628-6 against reference pictures. Chalking also lowers the gloss, which a glossmeter at 60° can follow over time (ISO 2813).

Telling it apart

Fading and discoloration: A little oil rubbed on the surface darkens a chalked film again, while true pigment fading does not darken.
Contamination and inclusions: Dust and dirt lie on top and wash off with water. Chalk keeps forming as long as the binder breaks down.

Repair

Chalk must be removed, by washing and abrading, before overcoating, or the new coat will not adhere. The specification and the coating manufacturer have the final word on the repair procedure.

20 practice items in the members area mention chalking (CIP 1: 7 · CIP 2: 8 · FROSIO written: 5).

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.