No matter what surface you are painting, you can end up with paint that is sagging. The most common cause of sagging is that you have simply applied too thick coats, i.e. the dry film thickness was too high. The coat does not have time to dry in time before gravity takes over. Another cause may be that too much thinner was added, etc., which leads to a change in viscosity. If you applied the paint/coating with a paint sprayer, you may also have held the nozzle too close to the surface, so that the coat was too thick (see reason 1).
The consequence of applying too thick layers can be that the paint dries skin dry, i.e. the outermost layer hardens before the rest of the coat has time to release the solvent. This traps the solvent gas in the paint and the coat does not harden for a long time afterwards. The paint feels soft and the adhesion is poor.
Correcting the mistake can be difficult, as the paint layer must be sanded down. If the film is thick and soft, this becomes challenging.
So avoid sagging by applying the coats at the wet film thickness specified in the paint's product data sheet and verifying along the way with a wet film gauge. It is better to apply more coats than too thick.
If you would like to learn more about the correct application of paint, please read our ABC of painting.
