Always wear gloves when mixing and using epoxy!!!
If you aren't allergic to epoxy you could become allergic over time. I don't use it that often but I still wear gloves. Using Epoxy is like doing fillets with running honey. You can be as careful as you want but it gets everywhere!
Like I mentioned, my experience with epoxy fillets is limited. Please don't comment and tell me: "That's not the way I do it!" This is simply my experience from other's posts on the Rocketry Forum.
Mix up small equal parts of part A and B.
You'll only be able to do two facing fillets at one time so you won't have to mix up very much initially.
Some builders add Micro Balloons to their epoxy mix. The small silica beads thicken the mix and make the fillets easier to sand after it dries.
To smooth out the fillet you can use the same diameter dowel you used to mark the fins with pencil. In this case I used a rounded popsicle stick that matched the dowel diameter.
Dip the stick into the purest alcohol you can find. This is 90% pure. Shake off the excess alcohol.
Run your dowel or rounded stick down the fillet for a few inches and wipe off the excess. If you tried to do the entire length of the fillet in one pass there will be too much excess epoxy at the end.
Epoxy runs and is self leveling. A big drop at the edge of the tape could run down and back into the fillet.