RE: cabosil vs micro balloons
Fine balsa saw dust mixed with West Systems epoxy works very well for certain applications where I'm looking for sandable fill in areas where polyester type fillers would dissolve the piece that I'm working with. This thin viscosity resin soaks into the sawdust nicely so you end up needing much less resin than anticipated.
I used to have a small tub of micro balloons that lasted me close to 25 years and now that it is gone...I could not care less.