Estes Mercury Redstone #1921 Build, Part 4, Fins C

I've explained a few times before how to sand sharp angles on scale fins.
On the Mercury Redstone, the leading edges are sanded to a wedge shape. To better see the "knife edge" thickness I like to blacken the leading edge. This gives a high contrast to the light beige balsa.

A permanent marker is used. But - you don't want the black ink to soak into the balsa. It would bleed through the paint coats.
Seal the edge to be marked with medium CA super glue. I buy mine at the Dollar Tree store.
Squeeze out a drop and pick some up on a Q-tip

Run the wet swab down the leading edge.
This seals the wood and will keep the ink from permeating too deep. It also strengthens the very thin sanded edge.

NEVER put CA glue down or near the root edge. Your white or yellow glue won't soak in.

Here's the top of two square fin rudders.
TIP: Think ahead! Which leading edge would you want at the top of a thin wedge shape?
Obviously the one on the right.

If the leading edge is too grainy, flip it over and run a CA coat on the closed grain side.

Here the CA'd leading edges get a quick pass with a permanent marker. A QUICK pass. You don't want too much ink to soak in.
The CA glue should keep the black ink on just the surface.

Don't worry, all the black ink will be sanded off in the end.