What is a double primed canvas?

[10ClarionSt]

I've seen Bob Ross telling viewers that he has a pre-stretched, double primed canvas. What has he done to it then? As you can tell, I'm not very arty!

[sandyRoe]

Coated it to seal the surface before painting?

[DTCwordfan]

There are four basic varieties of prepared canvas available in most artists' materials stores.

They are acrylic-gesso-primed cotton, acrylic-gesso-primed linen, glue-sized single-primed linen, and glue-sized double-primed linen. All of them can be used as supports for oil paintings. Acrylic or vinyl paints, however, may only be used on the acrylic-gesso-primed canvases, which conforms to the rule of not using a water-based paint over an oil-based surface.

"Single-primed" and "double-primed" refer to the number of coats of lead paint that have been applied. The more coats, the smoother the surface, the stiffer the canvas, and the greater difficulty in stretching it. "Double-primed" is less flexible than "single," which makes its permanency more questionable.