Chalking of resin based paints.
Cause
Overall or partial chalking and loss of gloss is due to the erosion of the paint film being exposed to UV light.
Certain colours contain pigments that are not light fast, therefore are more inclined than others to fade and chalk with age when exposed to ultra violet rays.
Wash down with sugar soap and water, rinse, allow to dry, sand down and paint with the required system.