The present invention relates to a process for preparing a surface treated filler material product with mono-substituted succinic anhydride(s) and a mixture of aliphatic linear or branched carboxylic acids comprising stearic acid, a surface treated filler material product, a polymer composition, a fiber and/or filament and/or film and/or thread and/or sheet and/or pipe and/or profile and/or mold and/or, injection molds and/or blow mold comprising the surface treated filler material product and/or the polymer composition, an article comprising the surface treated filler material product and/or the polymer composition and/or the fiber and/or filament and/or film and/or thread and/or sheet and/or pipe and/or profile and/or mold and/or injection mold and/or blow mold as well as the use of at least one mono-substituted succinic anhydride and/or salty reaction product(s) thereof in combination with a mixture of aliphatic linear or branched carboxylic acids comprising stearic acid and/or salty reaction product(s) thereof, for improving the flowability of a surface treated filler material product and for improving the dispersion of the calcium carbonate in the polymer matrix of a polymer composition.