CAUSES: Sagging (pattern in paint film which resemble curtains) has several possible causes. Application over a hard or glossy finish is a common cause. Other causes include applying too much paint, excessive thinning or application over a residential film left by a washing compound.
Sagging also can be caused by using too much solvent or by using solvents other than ones recommended by the coating’s manufacturer.
Paint cold surfaces will sometimes cause sagging. Do not paint a surface that feels cold to the touch. Do not paint when humidity exceeds 85%, as drying time will slower and this can cause sagging.
SOLUTIONS: Always sand glossy finishes before painting them. Liquid preparations designed to soften and clean hard coatings and available where sanding dust is a problem. To determine if the paint is being applied too thickly, measure the square feet covered per gallon of the sagging coat and compare it to the recommended spreading rate. If the rate is too low, increase it by applying a lighter coat or even by thinning the paint is necessary, however, this latter approach may cause sag due to the thinning. Sometimes in cold weather, paint stirred in garages or basements thickens. Instead of suing paint thinner in these cases, immerse the can in a pail of warm water to restore it to normal consistency. If sagging is noted during application despite all precautions, try brushing upward with log, even strokes, wiping the brush after each.