A new thickening additive, WEST SYSTEM Brand 406 Colloidal Silica, from Gougeon Bros. Inc. (Bay City, Mich., U.S.A.), is designed to prevent runs and sag in epoxy adhesives and fairing compounds. Often used in combination with other fillers, the 406 additive reportedly improves the strength, abrasion resistance and consistency of fairing compounds, resulting in a tougher yet smoother surface. The product comes in 1.7-oz containers, retailing for under $7.