Chalking
Chalking turns up in 2 fields: corrosion, powder coating. Here is what it means to the people in each one.
Chalking in corrosion
Powdery residue on an aging coating surface as sunlight breaks down the binder.
Weathering of a coating in which ultraviolet light degrades the resin at the surface and frees pigment as loose powder that rubs off on a finger or cloth. Epoxies chalk heavily in sunlight, which is why outdoor epoxies get a polyurethane or other UV-stable topcoat. Chalk is rated with tape or cloth tests and must be washed off before overcoating, or the new paint bonds to dust.
"Old epoxy on the tank is chalking bad, pressure wash it before the urethane goes on or it'll peel."
Chalking in powder coating
A powdery residue on a weathered finish where sunlight has broken down the resin at the surface.
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."