"Do no not grind the glow powder or try and dissolve it. On a microscopic level, the glow powder works by an energy exchange in its crystalline structure. These crystals are very small and just appear like a fine powder. When they are mixed with a clear medium as we do with our glow paint, the glow powder does not dissolve, it is just suspended in the medium. If you mix the powder with a medium that causes it to break down and dissolve (like water), the glow will be ruined. The same thing will happen if you grind the glow powder. You crush and destroy the crystal matrix and the energy will not be able to transfer between the glow material molecules causing a dramatic drop in glow brightness."
In my direct experience, grain size hasn't made much difference. Also, I use several kinds of very fine dusts/powders in my resin work and I've come to loathe the whole category. They go everywhere at the slightest puff of air, stick to equipment and tools, etc. I already wear a mask, but with chemical (NIOSH magenta or olive) rather than particulate cartridges, so there's that too. Of course YMMV. Perhaps also relevant is that I tend to work mostly with low-viscosity resins because it's easier wrt bubbles and edge creep, but I've had the settling problem even with higher-viscosity formulations. Maybe some day I'll hit on a combination of materials and techniques that solves the problem. Just hasn't happened yet. Good luck!
I've ordered some, will try a little comminution to get fine dust. At some point, viscosity should predominate over settling.