The first coat on a surface. In the old days this was usually quite different to the subsequent layers and served to seal the surface to prevent over absorption of the ground into the substrate. In the case of oil painting on canvas this primer consisted of Rabbit Skin Glue to be followed by a lead based ground. Acrylic gessos are usually excellent primers as well as ground so that the same material is simply applied in successive layers until the desired effect is obtained. Where the substrate is particularly absorbent, spraying or brushing on water first can be advantageous. The use of acrylics has eliminated the need for much of the elaborate surface primings once employed.