Definition - What does Sag Resistance mean?
Sag resistance is defined as the ability of a paint or coating to resist sagging failure, which is a bending or drooping of the wet coating. Sagging is a factor of the coating's composition and viscosity. A coating with a high viscosity has a greater sag resistance than one with a low viscosity. Sag resistance is usually determined in accordance with the ASTM D4400 - Standard Test Method for Sag Resistance of Paints Using a Multinotch Applicator.
Corrosionpedia explains Sag Resistance
Paints and coatings that are applied to vertical or slanted surfaces are susceptible to sagging. In some cases, sagging can occur in areas where there is a hole, bracket, corner or edge that catches more paint, resulting in an uneven coating distribution. In addition to being unsightly, sags can cause the finished surface to be uneven.
Paint and coating manufacturers make every effort to formulate products that are less vulnerable to sagging. While the type of paint can contribute to sagging, this defect can also be caused by improper coating application.