> — different thickening agents create different strength joints (e.g.,
> microballoons are great for thickening and fairing, but create a much
> weaker joint than cab-o-sil or woodflour).

That's true, but even with low-strength fillers like microspheres the material is much stronger than the wood, so it's a distinction without a difference. If the joint fails, it won't be at the epoxy.