Aerosil and Cab-o-Sil are competitive products but are basically the same, fumed silica. There are different grades, related to particle size and whether they are hydrophilic or hydrophobic. This relates to how well they work with different chemistries of resin and/or epoxy systems. There's some good technical info at the Cabot web site, but you have to know the details of the specific epoxy system you're working with to make sense out of it. I contacted the people at MAS because I have access to large quantities (10 and 20 lb. bags) of both brands of silicas here at work. They would not give me details on their chemistries but they stated that any of these products would work. I would recommend some test pours and bondings with the resin you're using to be sure. Dennis

> air-o-sil is a fumed silica, similar to cab-o-sil, however, there are many
> different grades. some grades are good to thicken epoxy. i would suggest
> two things: 1. get the number of the particular grade and contact the
> epoxy manufacturer or 2. test it in a small quantity of epoxy and see if
> it cures. if it does....... go for it.

> i used cab-o-sil in all of my fillets without any problems. this material
> does make the fillet somewhat whitish in color. good luck steve