Mix epoxy resin and hardener together at the correct ratio before the addition of fillers. The following fillers work well with System Three General Purpose Epoxy. Be sure to read The Epoxy Book for a complete discussion and guidelines on using fillers and additives.
This material is also referred to as colloidal or fumed silica, and is an excellent thixotropic agent which is used to control the viscosity of mixed epoxy systems. Silica can be used alone for a very smooth, non-sagging, high strength mixture or combined with fillers or fibers to make them non-sagging. A 5-quart quantity of Silica Thickener when combined with 1 gallon of mixed epoxy/hardener, will produce about 1.75 gallons of a non-sagging firm paste.
Wood Flour, a fine sawdust that has been filtered so that there are no lumps, is excellent for creating wood glue and structural fillers and fillets. It is a fibrous filler that is also thixotropic. To make a smoother fillet, some users add a small amount of Silica Thickener.
A finely ground fiberglass that can be incorporated into a structural filleting putties to improve tensile strength. The addition of Silica Thickener will prevent sagging or draining.
These bulking agents are hollow, clear microscopic spheres that make a low cost, low density filler. Added to epoxy resin and hardener mix, they make a good, heat-resistant, light-weight fairing compound with good compressive strength. Mixture can be blended with a small amount of Silica Thickener to prevent sagging. Not recommended for glue.