Things like wheat and wood flour add bulk but do not really stiffen the mix until you add so much that the end resin is weak. Filleting and fairing are two different things. When you are filling the joints of a stitch and glue hull to form a smooth even curve, that is filleting. When you are spreading a thin layer of epoxy on the hull with the intention to sand it down to a smooth even surface, you are fairing. For filleting , you want strength and a mix that will retain its shape until it cures.
Fumed silica is the filler of choice.
Because of the shape of the particles, it requires the least amount of filler to achieve the stiffest mix thus retaining the epoxy's strength. For fairing , you want as much volume as possible with the least weight and easy sandability. You want to keep the silica to a minimum because it makes sanding harder. There are a number of substances that add bulk to epoxy but do not increase strength or change viscosity very efficiently. Wheat flour, lime stone, gypsum and talc are among these fillers.