Estes Black Brant III, Part 6, Fin Taper Sanding

Use your thumb as a "stop" against the high point lines.
(For clarity I left the pencil shading off the surface of this fin.)

Sand a few stokes and take a look at the black ink left on the leading edge. Turn over the fin and sand some on the other side.
Always check back to the ink on the leading edge. Make sure the remaining ink thickness is consistent on both sides.
When the taper is done, a quick hit with 400 grit will take off any of the remaining black line on the leading and trailing edges.

Here's a finished fin.
The pencil shading help you see how straight the sanded edge is even if you take off some of the original pencil line.









On the left is a view from the root edge.
The inset picture shows the outside edge closest to you.