The West System 406 Colloidal Silica is an epoxy thickening additive used to control the viscosity of the epoxy. It prevents epoxy runoff in vertical and overhead joints. This is a very strong filler. It creates a smooth mixture, ideal for general epoxy bonding and filleting. It is also our most versatile epoxy filler. Often used in combination with other fillers, 406 can be used to improve strength, abrasion resistance, and consistency of epoxy fairing compounds. The result is a tougher, smoother surface.