Before applying any paint, the surface of the panel must be prepared by sanding, to roughen it, and by applying a layer of paint called a “ground”, a coating that bonds with both the panel and the oil colors. Until recently we used a procedure that has been used for hundreds of years, three coats of hide glue, applied hot, followed by two coats of oil primer. Today we use an acrylic “size” followed by an acrylic Gesso. We resisted the acrylic coatings because we were not certain that they would bond well with oil paint. However, paint chemists have informed us that good quality Gesso today is formulated to be porous, allowing the oil to make a strong mechanical bond with the Gesso even though the oil does not make a chemical bond with it.