Because canvas is a porous surface, paint can seep into its fibers like a sponge. Priming the canvas prevents this seepage. Besides acrylic gesso, canvas can also prepare with a oil ground. A size should be applied before oil ground. It is the material applied as a coating to protect the support from any deleterious effects from the ground or the paint. This is usually hide glue, vegetable or animal gelatine, acrylic polymer or some other synthetically derived product.