Sagging is the term used to describe all types of flow behaviors on vertical surfaces due to gravity which result in undesired appearances. (Figure.1) This defect stems from excessive fluidity (very low viscosity) and may develop during application as well as the stoving of the coating film. 

Fig 1. 

If the wet coating applied on the vertical surface is considered to be comprised of a large number of thin and parallel layers, the movement of these layers due to gravity that increases in speed with increasing distance from the application surface may transform into sagging defects (Figure.2). 

Fig 2.

Decisive variables for sagging are viscosity, surface tensions, wet coating thickness and density. 

It is apparent that what is meant by viscosity is the viscosity after the coating gets in contact with the surface. Hence, rheology additives that will enable a rapid increase in the coating viscosity following the removal of the share force applied by brush, roller, spray gun etc. are effective in sagging control. The fact that these rheology additives may delay the viscosity increase following the removal of shear (: thixotropy) will enable the optimization of sagging resistance and film levelling characteristics. On the other hand, the breaking off of the solvents from the film right after application leads to an increase in viscosity and the increase in the viscosity of the cooling coating is further enhanced due to the heat absorbed by the evaporating solvents. Therefore, taking into consideration latent heats of evaporation in addition to the solvent evaporation rates may provide interesting results when sagging is controlled through the use of the proper solvent. 

Fig 3.

Another parameter that can be taken into consideration when thinking about the sagging phenomena is the surface tension value which varies as the film loses solvent. An increase should be expected in the surface tension of the film with the evaporation of solvents as components with low surface tension. It has been illustrated through repeatable experiments that increased surface tension leads to an increase in sagging tendency provided that all other coating properties remain unchanged (Similarly, decreasing surface tension results in decreased sagging tendency). 

As a result, the higher the latent evaporation rates of the solvents used in the coating are; the higher the temperature decrease will be resulting in an increase in surface tension and accordingly in sagging tendency. 

Sagging tendency is directly proportional to the cube of wet film thickness. Hence, an increase by 25 % in wet film thickness leads approximately to the doubling of sagging tendency. 

Wet film density has a direct impact since it determines the force of gravity resulting in sagging behavior: 
G= m.g = v.ρ.g Here, G denotes the gravity force, m represents the mass, v is the volume and ρ denotes density. The change in film density due to the changes in temperature during the sagging process is negligible with regard to its impact on sagging tendency.