Air-o-sil is Colloidal silica( fumed silica, cab-o-sil...) and is used to increase viscosity and thixotropic properties (non sagging) of epoxy. Similar stuff to West System 406. It is used as the main ingredient in most fillers for filleting and bonding. It makes the epoxy more brittle and harder to sand but is the perfect general purpose thickener. Don't use it for clear coating! Microballoons are far weaker filleting compund but lighter. Use microfibers (shreaded fiberglass)for top strength structural filleting and bonding.

Vaclav OneOceanKayaks.com

> A friend gave me (five) one gallon cans of "air-o-sil" has
> anyone heard of used this product? I made a test batch and it seems more
> fluffy than microballons but the strenght seems to be the same, The color
> is very white compared to other fillers, I would like some advice on this
> before i build a craft with it.

> Thanks in advance!-Johnnie