Epoxy can be thickened with different materials for different applications. For many applications, a blend of fillers is required to achieve the desired physical and structural properties of the epoxy.
Microballoons: Lightweight, great for fairing applications, sag resistance, good for applications where sanding will be required, but not good for structural areas or filleting.
Colloidal Silica: A great thickening agent for structural bonding or fillets, ideal for increasing the viscosity of epoxy or making the epoxy hold a particular shape with structural properties. Harder to sand.
Milled Glass Fibers: Great for structural applications where the glass fibers help reinforce the area. Does not provide sag resistance or hold a shape.
Wood Flour: Good for structural applications and filleting; has some sag resistance.