Estes Black Brant XII, Kit # 9723 Build, Part 7, Shaping The Thick Fins TIP

I can hear the voices: "That's not the way I'd do it!" Well, go with what works for you. 

With this method it's easier to see a consistent width on the leading and trailing edges.

On the left is one of the lower, larger fins after lamination.

On the right is the fin edges trued up using 220 grit on a sanding block.

When shaping the diamond taper, it's difficult to judge the width of the leading and trailing edges on the light colored balsa. I blacken the edges for a better contrast.

Don't blacken the edges until you seal the balsa - the ink can't soak in and stays on the surface. 

Squeeze out a drop of medium CA on some scrap cardboard.

Dip in a Q-tip. Run the CA glue down the leading edge letting it soak into the balsa. Get a complete coverage down the leading and trailing edge faces. 

Now you can run a wide point Sharpie down the CA glue sealed leading and trailing edges. Go quick, use no more ink than you have to, to get a full black coverage.

The CA glue prevents the black ink from soaking into the balsa. 

Here I've started sanding the diamond taper. 

The black ink lets you better judge the thickness of the leading and trailing edge.

I haven't started sanding to the center high point, just the width of the top and bottom edges.