Brian,
406 is what I was thinking of. That's fumed silica. It's a terrific thixotrope and results in a very smooth finish. That's what I use as my surface coat in building molds. This material admixed into epoxy thickens the epoxy quickly
I have not used 410 but looks like it's a worthwhile material to have around.
Matt,
I will try various blends of this ,glass spheres and powder and see how it works.
It is compatible with the dope (does not break down) and makes a very smooth slurry for brushing.
It is very light.
We'll see !
Brian