Why coatings chalk
Ultraviolet radiation, moisture and oxygen break the polymer chains of the binder at the surface. As the binder erodes, pigment particles are no longer held and remain as powder. Some resin families are more sensitive than others. Aromatic epoxies chalk relatively quickly in sunlight, which is why they are normally topcoated with a UV-resistant finish such as an aliphatic polyurethane or a polyaspartic when exposed outdoors. Chalking on an epoxy used under a topcoat or indoors is mainly a matter of appearance and recoat preparation.
What it does and does not mean
| Observation | Usual meaning |
|---|---|
| Light chalking, film intact | Surface ageing, protection usually still present |
| Heavy chalking, loss of gloss and colour | Advanced erosion of the topcoat, film thinning |
| Chalking with exposed primer or rust | Film consumed in places, maintenance needed |
| Chalking on an epoxy exposed to sun | Expected behaviour of the resin, not a defect of application |
Chalking alone does not prove that corrosion protection has failed. It does reduce film thickness over time, so dry film measurements and an assessment of rusting per ISO 4628-3 or ASTM D610 complete the picture.
How it is rated
- ASTM D4214 describes several methods, including a cloth or felt rub and a tape method, with results compared against photographic references.
- ISO 4628-6 uses an adhesive tape method and a rating scale for degree of chalking.
The method and acceptance limit should be stated in the inspection plan so that results are comparable between inspections.
Before overcoating
A chalked surface must be cleaned before any new coat. Depending on the degree and the specification, this means washing with water and a suitable cleaner, scrubbing, pressure washing or light abrasion, then checking again that no powder remains. Compatibility of the new coat with the old system and a test patch with a pull-off adhesion test reduce the risk of delamination.
Recording the chalking rating at each inspection, together with film thickness and rust grade, gives a trend that helps decide when to clean and overcoat rather than strip and recoat.
For repainting existing structures, see our maintenance and rehabilitation service.