OK, here is how I do fillets. With Aeropoxy Light, you have to stir it up real well. I made up a wire whip and put it in my cordless drill and stirred away. The hardener has a "syrup" on the bottom that has to get stirred in thoroughly or you'll get a rubbery fillet that won't sand. I've included some pictures of the stuff all stirred up. The hardener will look like melted milk chocolate when stirred properly.
I use a 10X multiplier on my gram scale so I can measure out a 1/10 of a gram accuracy. I put the mixing cup on the adapter and turn on the scale. When it reads 0 then its ready to go. For the fin and each side of the stab, I used 4 grams of the filler and 2 grams of the hardener. I know this is too much but when you buy it by the 1 1/2 pounds, you can be a bit wasteful!. I then mix it up real well and using a craft stick I trowel it into the corner pushing the Aeropoxy into the crevices. When that's done I use my fillet tool. I glued on a 1/4 inch ball bearing to a 3/16 dowel. I use the ball bearing because it gives a constant radius no matter what angle it is against the side. Once I form the fillet with the tool, I scrape off the excess with a single edge razor blade. I then fold a paper towel into quarters and soak the towel with 91% rubbing alcohol. I then clean up the sides/stab/fin etc by running the edge of the paper towel very close to the fillet. Once cleaned off I let the fillet sit for about 45 to 60 minutes to set up some. I then dip my finger into the alcohol and run back and forth along the fillet very gently. This action will smooth the fillet and feather the edges very nicely. Again the fillet get the wetted paper towel to clean up all around the fillet. I then let harden over night. I'll sand them when they are all done.
Tonight I got the fin and stab done. I also did the first application on the canopy too.