Finely powdered stone fillers for polyester resin. Please note that we cannot guarantee colours, and that enough should be in hand of the same shade for the whole job in hand.Fillers are used for a variety of reasons: for effect, as with metal fillers, slate or synthetic onyx; to improve opacity when using pigments; or to bulk out the resin for economy and to reduce heat (exotherm) when casting large solid pieces. Other useful properties, such as improved hardness, scratch resistance and thixotropic qualities, can also be achieved by adding fillers.