Originally Posted by az_gila
...how do the chopped up cotton fibers of flox compare to the tumbleweeds of cabosil for bonding strength?
I built a fiberglass airplane and used lots of flox and micro in various places. The micro (epoxy and microballoons) was a lightweight, sandable, non-structural filler. Besides filling low places in the exterior, it was used when making ribs and bulkheads out of foam and glass cloth. The micro was squeeged onto foam pieces before applying the fiberglass cloth; It was worked down into the foam so as to make a better bond with the glass cloth. Micro has medium compressive strength - better than West Systems and microballoons, but not as good as flox.
The flox (cotton fibers and epoxy) was a hard, semi-structural filler which could be shaped with a Dremel tool. It has good compressive strength and abrasion resistance. I used flox to attach my wing ribs to the skins (with bid tapes over the joint). I used structural adhesive (3M 2216 or Hysol EA-9430) for critical joints. Where runout was a problem, I mixed a little flox in with the structural adhesive.
If you Google "Burt Rutan composite construction" there are some good How-To videos on YouTube.
The only place I used Cab-o-Sil and epoxy was to paint the inside of my lower cowling to provide a nice, smooth, cleanable surface that would shed oil. If I had it to do over, I probably wouldn't have done that because the Cab-o-Sil and epoxy is heavy. It's my sense that Cab-o-Sil is intended as a thickener only - at least the company that makes it doesn't advertise it as structural. It's used to keep epoxy from running or draining out of joints.
I found this pretty good summary of the common FILLERS & MODIFIERS