The West System manuals are a great read on epoxy use. Different fillers excel at different tasks. Fibers (cotton or glass) are good for gluing because they trap some epoxy to help prevent a dry joint from over clamping. Fumed silica (Cabosil) make good smooth strong fillets and prevent sagging on vertical surfaces but is impossible to sand. Mico balloons (glass spheres) are for non-structural fairing because it sands easily and has no shear strength. Wood dust (sanding dust) is cheap and is wood colored but is not as strong as fumed silica or as easy to use for fillets.

http://www.westsystem.com/ss/filler-selection-guide/