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To achieve desirable properties in composite components, adhesion between fiber and matrix must be optimized. Adhesion requires sufficient saturation with resin (wetout) at the fiber/matrix interface. To ensure good adhesion, attention must be given to fiber surface preparation, such as the use of a surface finish or coupling agent, often termed sizing. Sizing, applied to glass and carbon filaments immediately after their formation, actually serves three purposes: As it enhances the fiber/matrix bond, it also eases processing and protects the fibers from breakage during processing. Although it accounts for only 0.25 to 6.0 percent of total fiber weight, sizing is a dynamic force in fiber reinforcement performance. Sizing chemistry distinguishes each manufacturer's product and can be optimized for manufacturing processes, such as pultrusion, filament winding and weaving. For example, developments in sizing formulations have variously resulted in more cleanly chopped glass with reduced fuzz, glass that wets out more efficiently, and glass fibers that contain no chromium compounds.

Historically, carbon fiber was sized only for compatibility with epoxy resin. Today, fiber manufacturers are responding to demands from fabricators and OEMs to produce carbon fiber forms compatible with a broader range of resins and processes, as carbon fiber use increases in applications outside the aerospace arena.