The Importance of Sizing
Sizing the canvas is an absolute necessity when working in oils. Oil paint should never come in contact with the fibers or the canvas will "rot," eventually becoming weak, brittle, and crumbly.
Hide Glue: The purpose of hide glue is to protect the fibers of the canvas from coming in direct contact with the succeeding applications of oil priming. The acids contained in the oil vehicles is known to breakdown and weaken the canvas. When applied to a dry stretched canvas, the canvas will begin to tighten while it is drying but may tend to sag when completely dry, depending upon the quality of the canvas. At this point you may want to peg stretcher bars to restore the desired tightness.
Conservationists caution paiters about hide glue. Hide glue absorbs atmospheric moisture on damp days and swells, and gives off moisture on dry days and shrinks. Over time this movement of the glue can cause paintings to crack. An alternative to hide glue is a neutral pH PVA (Polyvinyl Acetate) size.
There are many commercial brands and types of hyde glue that are used, but to ensure proper coverage and application, follow the instructions of the particular brand you purchase.
Acrylic Canvas Primer: This is typically called "gesso", it is not. While most commercial "gesso" brands are above the acid level pH, they are assumed to be safe for applying directly to the canvas. However, in most cases the addition of a high pH ammonia is added, which evaporates on drying. In the dry state the priming may be acidic and harmful to the canvas fibers.
If desired the canvas may be slightly sanded to provide a smoother surface for painting. Unlike the glue sizing, it is possible to apply a heavier layer of acrylic sizing as it remains permanently flexible and improves the overall flexibility of the completed ground.
PVA Size: The advantage of a PVA glue size is its stability. PVA size does not absorb atmospheric moisture like rabbitskin glue does. This elimnates the swelling and shrinking that can cause cracks in the painted surface.
One disadvantage to PVA size is that it does not tighten the canvas like rabbitskin glue.
