There are various reasons for sagging. Picking up too much paint with a paintbrush and applying a thick coat can result in paint drops travelling downwards along the surface. The consistency of paint can be affected in cold or humid weather and can result in sagging. Diluting a paint with a thinner can change its viscosity and make the paint sag. Sagging occurs when airless spray painting is done either with the wrong spray tip or by holding the gun very close to the surface.
Always maintain the required spread rate. If you feel that the paint coat is thicker than necessary, spread it out when still wet with a brush or roller. If you have noticed sagging after drying, wait for it to dry completely before sanding and repainting with good-quality paint. Avoid diluting the paint with thinner. Most good-quality paints do not require further thinning. If the weather is too cool or humid wait for a later time to do the painting job, as the paint may not dry at the required rate.