As long as an artist doesnt lay on casein too thickly, its an excellent choice for underpainting because its insoluble once it dries, explains Ross Merrill, chief of conservation for the National Gallery of Art in Washington, D.C. The casein will easily accept the oil and will form a good mechanical bond.
One thing you must never do is underpaint oils with acrylic paint, says Merrill. Acrylic paints are formulated with a closed surface to protect the paint from stains, grime and other pollutants. This closed surface is nonabsorbent and wont form a good mechanical bond with oil. Over time, the oil on top could actually delaminate and come off in layers.
Using acrylic gesso as a canvas primer works fine with casein or oil paint because it has an open surface to accept the paint layer and form a mechanical bond.
Mark Gottsegen is an associate professor of art at the University of North Carolina at Greensboro and chair of the ASTM Internationals subcommittee on artists materials.