UV breaks the binder at the film surface, freeing pigment so it rubs off as a dry powder on a finger or a dark cloth. Epoxies chalk fastest, often within months outdoors, followed by hybrids; polyesters and fluoropolymers resist far longer. Chalking is rated by rubbing a cloth or tape on the surface and comparing it with a numbered reference.
Black epoxy rails are chalking already, wipe your hand on them and it comes off gray.
Also heard as
- chalk
Related terms
Epoxy powder
Powder based on epoxy resin, prized for chemical and corrosion resistance but weak in sunlight.
Hybrid
Powder that cures an epoxy resin and a polyester resin together, trading some outdoor life for low cost and good flow.
Superdurable
Describing polyester powders built with weather-stable acids that hold gloss and color several times longer outdoors.
AAMA 2604
The mid-tier North American architectural coating spec, requiring five years of Florida exposure with limited fade.