RE: v-tail fillet
30 minute epoxy would work,but it is not going to be all that easy to sand..
I use a product called SuperFill, I got mine from aircraft spruce and specialty. Its not that expensive and it lasts forever.
It is also very light, and very easy to sand.
I lay a strip of tape about a 1/2 inch from the corner you are going to fillet, on both sides.
Mix up the product and work it into the corner with my finger.
Some people use a marble, ball bearing, or other tool to shape it, for me my finger works pretty good
after it starts to setup, then I take alchohol on my finger and "mold" the shape better, smoothing it out
Let it set a bit longer then pull the tape. The fillet should be firm but not cured. Normally I wait about a half hour
then using my finger and alchohol, I work the fillet further, blending the edges where I pulled the tape up so the blend into the surface smoothly
This minimizes the sanding needed.
Let it cure over night, sand with 180 rolled up into a tube working diagonally across the surface edge into the body of the fillet to blend it
then finish with 240 or whatever you feel appropriate.