Why Can You Paint Oils on Latex Gesso?
Latex gesso is a primer used to prevent the canvas from soaking up the paint. Today's gesso is quite different from that used in ancient times. Today the gesso is inexpensive enough for most artists to use. Gesso is water based and there's a rule of thumb to follow when you attempt to mix the two different mediums. Oil paint should not go on top of latex gesso.
History
Gesso found it's way to the art supplies years ago. Since the name gesso is Italian and means "chalk" you'd suspect that gesso came from Italy, but archeologists found earlier evidence of the use of gesso in ancient Egypt. The Italians, however, used gesso to build raised work. Later, when man began to create paintings on wood it proved very useful. It provided a protective layer when they primed the wood with gesso and prevented the wood from absorbing the paint. Originally, it was a mixture of whitening (chalk) and gelatin or glue made from animal skin. After mixing the ingredients, the artists then heated the mixture and the result was gesso. In 1955, modern gesso found it's beginning.
Considerations
Use the modern latex gesso because it's a lot more flexible than the original. The gesso of old tended to become brittle when it dried. Today gesso remains flexible and dries quickly. However, this flexibility and quick drying ability is often one reason that you can't paint over the more brittle, slow-drying oil paint. Gesso often acts as a primer for the canvas. Many commercial canvases have gesso already applied as the base
Time Frame
Understand that oil based paints take a long time to dry. Once the oil-based paint dries, it actually cracks and crystallizes. Even though the oil-based paint feels like its dry when you touch it, it isn't. Oil doesn't dry fast so the paint doesn't shrink rapidly like faster drying latex gesso.
Warning
The gesso doesn't adhere correctly to the oil based paint either. Just as it's counterpart, acrylic paint, the latex gesso doesn't adhere. While the process takes years to find cracks and chips, the painting deteriorates faster than those paintings that don't have latex gesso painted on top of the oil base. Sometimes painting latex gesso over oil causes separation of the layers. Occasionally, artists use mixed mediums and don't follow this rule. The painting looks great when at the time of sale, but years later breaks down and there's no recourse for the buyer.
Prevention/Solution
Use a pumice stone or sand paper to sand away all the oil base paint first before you apply latex gesso. Many artists find that by reducing the canvas back to the primary level, they successfully paint over the canvas with gesso. The most important factor is the removal of most of the oil-based paint.
