Add fillers to thicken the expoxy to the desired consistency for a specific job. Unmodified resin / hardener has the consistency of syrup and runs off vertical surfaces (suitable for coating and wettting out porous surfaces). By adding filler, a "thicker" consistency can be achieved (suitable for filling cavities, fillets, making large, uneven bondings and for use as a filler). 406 Colloidal Silica is a high-density silica flour filler for bonding, gap filling and filleting. Collodial Silica 406 is a thickening additive for producing an epoxy mixture specific to your needs.