Todd,
Ed is right. Your trouble is probably mixing the glass fibers with your first batch of filler. I use #1099 surfacing resin, which won't run or drip, from
http://www.fibreglast.com but resin and Cabosil works just as well. BUT... make your first layer without the fibers. Cabosil can easily be made thick enough where it won't run, (or use the surfacing resin) but save the fibers until later.
I know you know this, but it's key...
Whatever you use, take as much care as you can, when you mix it, to avoid introducing air bubbles. An impeller-type mixer, which goes to the bottom of the cup, is the best way to mix. Tap the container of mixed "goop" on the table to loosen bubbles, let it exotherm for a few minutes, and when you apply it, really press it into the corners tightly, to remove bubbles. (I typically use a gloved finger, or a popsicle stick to press the stuff into the corners.
One other thing...
I recommend applying the next layer, whether it's cloth or resin, before your corner fillet has cured, so that you get chemical bonding. (It would be difficult or impossible to scuff a cured fillet enough to bond mechanically, without scuffing the surrounding mold surface, PVA, wax, etc..)