Fumed Silica—sometimes referred to as Cab-O-Sil (registered trademark of Cabot) or Aerosil (registered trademark of Degussa)—is a thickening agent that may be added to epoxy, urethane, or polyester resins to increase the viscosity. High filler loading will produce paste-like consistencies, ideal for creating fillets, repair materials, or making fairing compounds smoother and easier to apply.