Today many oil painters simplify and speed up the work of preparing and applying the size and ground layers by combining both into one ingredient--an acrylic emulsion ground (commonly known as acrylic "gesso")...

Acrylic gesso can be applied with a 2-3" wide gesso brush (or a soft housepainter's brush). Rather than applying one thick coat, it is better to apply two (or three) coats and sand the first one (or two) lightly. For the first coat, some brands can be thinned with water (don't use more than one third water; an overdiluted film won't bind properly), or thin with a combination of water and acrylic medium. Keep the brush damp.

Acrlyic gesso is popular with oil painters because it is applied easily, provides a a light reflecting surface, contains no toxic ingredients, and the brush is quickly cleaned with soap and water. It should be noted, however, that some experts question its acceptability as the primer underneath oil paints applied to a large, flexible support, such as a stretched canvas. The final verdict is still out, but critics point out that acrylic gesso remains flexible whereas oil paint becomes more brittle as it ages. Consequently, the two layers may ultimately be in danger of separating.

[from Robertson and McDaniel, Painting as a Language, pp.23-24]