Cab-o-sil is thixotropic which means it will not sag when you mix up a mayonnaise like consistency. It's extremely light so for a lot of fillets, weight is a consideration.

read this site and you'll be floored that it is used in foods!

http://www.eagerplastics.com/cab.htm

In the last several years I mostly use wood flour which I buy and it's finer than any saw dust I can make - and it's so cheap. I feel If I breath that in, I won't die as quickly as with Cabosil. I've used talkcum powder and corn starch too. They work fine but in a filet it will sag. So if you make a nice clean fillet and come back an hour later, it may sag and run. But for thickning up a glue joint, talc is fine. I always have some Cabosil on hand for when I want a stiff fillet.

I always always have some of West's low density filler too - if I want to fair something out that I'm going to paint. It's much easier to sand.

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