Primer or anti-spread is used to treat fabric so that the flow of paint is inhibited. This allows free hand painting with fluid paints such as Deka Silk and Jacquard Dye-na-flow. Control the flow of paint by using gutta outliner to form a resist around outlines or treat the silk fabric with a primer to make the surface less absorbant and more similar to paper.
Find out here how one beginner silk painter got along with using gutta outliner for the first time