Primers are painted onto the silk and dried, before the dyes or paints are applied. They keep the dyes/paints from spreading when they touch the silk, which normally has a unique wicking action that causes liquids to spread way out. Allows for freehand painting without Gutta or other resists. Think of them as a starch-like sizing to prepare the canvas with. They wash out in the end. Important: some work only with silk dyes, some work only with silk paints.
We have included Dextrins in this category because we didn't know where else to put them. You can do some really cool surface design with them.