For the scarf joints in my CLC, I thickened the epoxy with a mixture of about 2/3 silica and 1/3 woodflour. The resulting joint was very smooth, extremely durable and the wood flour provided a color closer to the color of the wood. (And everything I've read says to paint the joint edges with unthickened epoxy before applying the thickened epoxy...another trick is to put some drafting mylar over the joint before clamping it...the resulting joint is much smoother...)

From what I've read, the microballoons should only be used where you (a) don't need strength/bonding and (b) weight is a critical factor. Someone's earlier analogy of "think balloons in concrete" was very apt. The microballoons I obtained were the phenolyic (sp?) and the resulting mixture was dark red. It was smooth, but I wouldn't recommend using it on any of the external surfaces unless you're planning to paint over it.