GOLDEN Laboratories conducted a study to find out which medium used as a size would stop oil penetration the best. The results were that two coats of GAC 100 did the best overall, with Fluid Matte Medium also performing well. When using any of these products to block oil penetration, it was found that a minimum of 2 coats was required to adequately block the oil, with additional coats offering additional insurance and protection of the support. A typical Gesso required 3 or more layers to fully prevent the oil from passing through to the support. The reason for this is that the Gesso is designed to be porous and moderately absorbent to facilitate adhesion of subsequent paint layers.
For maximum oil blocking, let dry for a minimum of 3 days before painting.
To date, the best system to both stiffen the canvas and block oil penetration is to first apply 1 coat of GAC 400 to the front of the canvas to develop the stiffening of the fabric, followed by 1 coat of GAC 100 to achieve the oil blocking properties. Once dry, apply 2 coats of Gesso to the front.