The primer is the part of a piece often painted or sprayed on as a protective layer to prepare a surface for art. Manufacturers and artists use a ground on supports intended for art use.
Gesso and sizing are popular primers used in art projects. Companies such as Golden create a variety of gessos depending on the needs of the artist.
Located between the support and ground or medium, this layer should be in just about every archival art piece. Without the ground, the medium will often eat away at the fibers of the support over time.
Manufacturers often spray paper or canvas to prepare and protect the actual surface of the piece. If the manufacturer didn’t or they did but it is a lower grade product, the artist can prepare their own surface.