West System 403 Microfibers are a fine cellulose-based thickening and reinforcing filler designed to blend seamlessly with epoxy to create exceptionally strong, high-density adhesive mixtures. Ideal for bonding wood, fiberglass, metals, and a wide range of other materials, 403 Microfibers are commonly used in structural filleting, gap-filling, and lamination where superior adhesion and load-bearing performance are essential.