use micro balloons or wood flour something like that to thicken things you will have to sand. It will work much easier than cabosil. Cabosil can be brittle when used as a thickener by itself. Milled fibers will add more tensile strength and should sand easier too. Even some cardboard from a file folder and wax paper can be used as a dam for a form.
I typically include some fiber if I build up an edge. Micro fiber as suggested is good. I have also just glassed a thin strip or two of 1.5 oz mat (depending on how much it needs to be built up) on the corner, and then sanded that to the proper shape. I feel that the fibers will help keep the edge from chipping if it gets hit by something.