Materials (usually a starch solution) added to the surface of a partially dried paper web as a means of sealing surface fibers and increasing sheet strength and resistance to stresses. Additional substances may be used in addition to or instead of starch to impart other desired surface properties to the paper, such as increased water repellency, or resistance to penetration by other types of fluids, such as ink, oil, or grease. Surface sizing also reduces fuzz, loose paper fibers that cause blanket piling on offset presses. Surface sizing is added using a size press, a set of two rollers located near the end of a papermaking machine's drying section. As the still-damp paper web passes through the nip of these rollers, the sizing solution coats the surface of the web. Surface sizing differs in application and purpose from internal sizing. The addition of surface sizing depends upon the ultimate end-use requirements of the paper and the printing process for which it is destined. Paper that is to be run on high-speed web presses requires no surface sizing, as ink must be absorbed rapidly by the paper. Sheetfed offset printing may require paper that is surface-sized, to decrease blanket piling. Surface sizing that is added by running the paper web through a tub of the sizing solution is called tub sizing. Surface sizing is also called external sizing. (See also Sizing and Size Press.)
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