I agree--it's super light in air, but when you mix it with epoxy, it doesn't displace any epoxy to speak of. The resulting mix is as dense (unit weight) or even denser than solid epoxy alone. Wood flour and sawdust displace some epoxy (that which isn't absorbed by the wood fibers) and make the thickened epoxy less dense. Microballoons (glass and phenolic) are mostly hollow and really displace epoxy mass. While they feel heavier, they displace more epoxy molecules and actually make the mixture lighter.

When you mix cabosil into epoxy, you don't change the volume of the original epoxy mixture much. You've added more mass, but kept the original volume, making the mixture denser and heavier. Notice when you're mixing it that you put a pretty substantial amount in your mixing cup, but it all gets mixed in, and the downy appearance is all gone. The thickened epoxy remaining is about the same volume as your unthickened epoxy, but it's got a bunch of cabosil particles packed into it.

When you mix less dense fillers into epoxy, you increase the overall volume a great deal while only adding a little weight. The new mixture is less dense and lighter. (While it may not be as flowable, it is less dense--less weight per unit volume).

A molecule of cabosil is as heavy as a molecule of glass fiber. The cabosil particle, though, is only a few molecules thick, and doesn't noticeably displace epoxy. Your glass fibers are just larger particles of silica (which is denser and heavier than epoxy) so they pack a lot more weight into the volume of epoxy that was displaced.

That's why cabosil-thickened epoxy is harder to sand than microballoon-thickened epoxy. It's denser and harder.

I've heard of folks using portland cement as a structural thickener. Cement is a fine dust and looks almost as fluffy as the cabosil (it is about as fluffy as microballoons) but it really makes a dense mixture.

Shawn

: Hi Shawn,
: The Cabosil (silica thickener) I use is about a light as goose down. It is
: fluffy stuff, 2 ounces would fill a quart container. It makes a light
: fillet. The microballons I have (phenolic) are MUCH heavier than the
: silica thickener. Glass fibers are the heavy filler I have used.

: Larry C.