Size Untreated supports, flexible and rigid, are usually too absorbent to permit the uniform application of paint. Therefore, intermediate layers, called size and a ground, are first applied. Size is a thin solution, often a dilute glue, that is brushed directly onto the support. The solution permeates the support surface without coating it. It provides a good mechanical bond between the support and the subsequent coatings.
Most supports are unevenly absorbent.
It is difficult to predict the results of a paint film laid on an unevenly absorbent surface. A ground will ensure a particular kind of paint will perform with reasonable predictability. The paint on top of the ground can be a very thin single layer or multiple layers. Each of these are made up of natural or synthetic pigments suspended in a particular binder. Natural pigments are those which are mineral, vegetable or animal in origin.