I am Tosk wrote:
I believe silica is just a thickening agent to help prevent
sagging on vertical surfaces.

Silica is more than a thickener, it's a thixotropic agent.

This means that the resin+silica fluid has a lower viscosity when you
stress it (ie when painting or spraying or mixing). When you leave it
alone, the viscosity will increase a lot (quickly), making it a kind of
gel which will indeed prevent sagging.

With a thickener like wood flour, the mix will still flow when set on a
non-horizontal surface (albeit more slowly). You'd have to add
unworkable (with a brush) amounts to slow down the flowing to the point
where it stays put until the epoxy cures.

The silica is kind of a miracle thickener which "knows" what it should
be doing, but, as mentioned in the thread, the end result is hard to sand.

I think it's quite usual to mix in both silica and another thickener
like micro-balloons or wood flour, to adjust the viscosity properties
and the end density and hardness to what you are doing.

jf