This is the thixotrope which makes the resin and gelcoat non drip, thixotropic resins will have between 0.75 - 1% by weight included in the formulation and gelcoats around 2.5% .
Its not that easy to disperse this without a mechanical stirrer - it will go in if you mix it in by hand but it wont develop its full effect if not well mixed.
Technical Questions
Can i add this to a polyester resin to thicken it up a little?
Yes you can add cabosil to a polyester resin or even a vinylester or epoxy resin to increase viscosity.