Fillers can be used for modifying characteristics of a resin system. Microballoons will increase viscosity while reducing weight and strength. Fumed silica increases the viscosity. Cotton flock increases the viscosity and the strength of the resin system. The use of fillers is not usually a precise science. Whichever filler you select can be added until the desired effect is achieved. Microballoons are a common filler, and when used in large quantity, make a lightweight syntactic foam. This can be used as a surface filler and molding putty.