Using 400 grit on a block, lightly square the top of the edge taking off the remaining black ink.
NOTE: My fins don't actually taper to a real knife edge. the leading edge is squared off a little. If the fins were to go to a very sharp edge, the leading edge is weakened.
For now, leave the tape on for filling the balsa grain on the taper.
If the filler is sanded properly, it'll actually accentuate the leading edge line definition. After all that work, there's no reason to sand off the sharp edge now.
I'm filling just the leading edge taper first.
This is thinned Elmer's Carpenter's Wood Filler brushed on the leading edge above the masking tape line.
After sanding, the tape is removed. You can see how crisp the taped fill line is now.
next, fill and sand below where the taped edge was.
NOTE: Keep the filler OFF the root edge and out of the recesses for the fin mounts and body bands!
If you get a little filler over the taper line, carefully sand it off trying to keep that line straight and sharp.