A feather-weight thixotropic powder, which is used to thicken resins & gelcoats, add more to create a non-sagging filler that is lightweight and very hard. Also used to control the viscosity of resin and prevent runoff in vertical surfaces. Colloidal silica creates a smooth mixture, ideal for general bonding and filleting, so it's a very versatile filler. Often used in combination with other fillers, it can improve strength, abrasion resistance, and consistency of fairing compounds, resulting in a tougher, smoother surface. Add 3% to 8% by weight to the resin / hardener mix.