Available fillers and additives give you the abiltiy to customize WEST SYSTEM 105 Epoxy Resin for your specific application.406 Colloidal Silica is a thickening additive used to control the viscosity of WEST SYSTEM Epoxy and prevent epoxy runoff in vertical and overhead joints. 406 is a very strong filler that creates a smooth mixture, ideal for general bonding and filleting. It is also the most versatile filler. Often used in combination with other fillers, it can be used to improve strength, abrasion resistance, and consistency of fairing compounds, resulting in a tougher, smoother surface. Off-white. 1.7 ounces. For the suitability of fillers for specific applications, see theFiller Selection Chart.