I've built quite a few S&G using only wood flour as the thickener. I don't buy the wood flour, I save all of my sanding dust from various projects. The fumed silica can be used with, or in place of, the wood flour, and makes a harder, smoother fillet. The microballons make a weaker more sandable fillet, or fairing, material. I do use all of the materials at different times plus 1/32" milled fibers added to some mixtures for gluing and added strength. However...you can certainly get by with wood flour only. I have used screened sawdust, but it's too coarse.