What is the difference between sizing and priming?
Size is a glue barrier that prohibits the ground and paint from contact with and penetration into the fabric. In the case of rabbit skin glue sizing, it adheres all the fibers to each other so that they expand and contract together and react to environmental changes as one unit rather than each thread acting independently. When different areas of the painting react separately to the environment, then you get cracking of the paint film, and similar problems. Priming, also known as the ‘ground’, or ‘gesso’ is an absorbent coating which provides the paint a porous, reflective surface to adhere itself to. The gesso is not a size and will not seal or create a barrier: just the opposite it is absorbent.
