Definition - What does Skinning mean?
Skinning is a process in which paint or other form of coating in a container forms a thin solid layer on top of or in the liquid paint or coating. Skinning is typically undesirable, and the layer of paint skin should be removed prior to use.
Water-based paints and coatings are typically more vulnerable to skinning than paints that are oil-based.
Corrosionpedia explains Skinning
Skinning can occur through many ways. One way that skinning can occur is if a bulk container of a paint or coating is left opened. Over time, moisture is released to the atmosphere by the outermost layer, which causes the liquid to solidify. This can happen if the container is left wide open or if the lid is simply not tightly sealed.
Skinning can also occur even if the container is closed completely when temperature changes occur. For instance, if the ambient temperature rises, the container will increase in temperature faster than the coating. This temperature difference will cause the moisture to move towards the cooler main body of coating, leaving the coating that is left on the lid without as much moisture, thus causing skinning.
Airtight containers should be used to prevent skinning. Anti-skinning agents are also available.