What Is a Plaster of Paris Primer for Oil Painting & Acrylics?
Plaster of Paris primer, commonly referred to as gesso, is a traditional primer used for both oil and acrylic paints. Ground from gypsum, plaster of Paris is a fine white powder, calcium sulphate hemihydrate, CaSO4 2H2O. When mixed with water, rabbit skin glue and titanium white dry pigment, plaster of Paris forms a thin, spreadable paint. It dries to a reflective, absorbent coating, desired by artists but prone to brittleness and cracking.
History
Artists have been using plaster of Paris since the Anatolian and Syrian civilizations, more than 9,000 years ago, although it didn't receive its name until the 1700s. Following the Great Fire of London, in 1666, King Louis XIV of France ordered all wooden walls and doors in Paris to be covered in plaster, for fireproofing. Paris' location on top of a large gypsum deposit, the main ingredient in plaster of Paris, made it a natural center for the thriving plaster industry.
Benefits
Plaster of Paris primer provides an effective barrier from oil paints that can leech into and weaken canvas fibers. It dries to a smooth surface that can be sanded between coats to a velumlike finish. Gesso has a fine "tooth" that draws paint off the brush and forms a secure bond with acrylics.
Left over plaster of Paris primer can be stored dry and reused when reconstituted with water.
Drawbacks
The Absolute Astronomy encyclopedia describes gesso as a "permanent and brilliant substrate, as long as it is used on wood or masonite…(but) rather brittle and susceptible to cracking, thus making it unsuitable for priming canvas."
Acrylics and Oils
Acrylic paints require acrylic primers, of which plaster of Paris is one, since it is water soluble. Most modern acrylic primers are made with calcium carbonate and acrylic polymers, for added flexibility on canvases. Two coats of acrylic primer, even on pre-primed canvases, are recommended for acrylic paints.
Debate remains over the best primer for oils. Technicians at Golden Artist Colors, New York, emphasize that acrylic primers should be allowed to dry thoroughly before applying oil paints.
Others contend that rabbit glue sizing is the only time-tested primer to assure permanent physical and chemical bonding between oils and canvas. Acrylic gesso primers are considered unproven by curators at the Smithsonian Museum, who will not use them under oils, due to delamination concerns.
Plaster of Paris Primer Recipe
Dry ingredients
1 tbsp. pure cane sugar or rock candy (some recipes use honey)
1/2 tsp. of Armenian bole or red (or blue) dry pigment
8 tbsp. calcium sulphate hemihydrate (plaster of Paris)
2 1/2 tbsp. titanium white dry pigment
Wet ingredients:
10 tbsp. pure distilled water
1 1/4 tbsp. of rabbit or fish glue (seccotine)
Mix the dry and wet ingredients separately at first. Then blend them into a spreadable consistency.
Tools:
Resealable can or jar (not plastic)
Rubber gloves	
Sink or tub
Paper towels/tablecloth
Stir sticks
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