Sizing

Sizing is the application of a substance, such as hide glue or acrylic polymer, to a support to reduce the absorbency of the support and to keep the paint from coming in direct contact with the fibers that make up the support. The oil in oil paint will cause unprotected natural fibers to rot and become brittle. Polymer emulsion paints will not adversely affect natural fibers and can actually help preserve them. Although sizing is not needed to protect a support fromacrylic or vinyl emulsion paints, it is often used to reduce the absorbency of the support because, if too much of the paint sinks into the support, the color loses its intensity and develops a dull appearance.

The sizing used by the manufacturer of paper and board to produce a specific working surface should not be confused with the sizing needed for paintings with oil or acrylics. Watercolor paper, for example, is sized to help keep the color on the surface, not to protect it from the paint or to make it more permanent. All such sizing incorporated in paper and boards during the manufacturing process is not adequate for the preparation of supports for oil, or polymer emulsion, paints.