Details: 406 Colloidal Silica is an epoxy thickening additive used to control the viscosity of the epoxy and prevent epoxy runoff in vertical and overhead joints. This is a very strong filler that 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, resulting in a tougher, smoother surface. Color: off-white.