Sagging happens when the paint droops downward after being applied on the surface. It is caused by the pigment separating from the paint and settling at the bottom of the container and comes as a result of insufficient stirring or shaking during storage or storing for too long or under too much heat or faulty thinning.
Avoid storing in hot locations for long periods. Store in accordance with the manufacturer’s recommendations. Thin only with appropriate recommended thinners.
Rub down the sagging parts and recoat carefully