Wolverine Rocketry Phoenix AIM-54A Build Part 13 Lower Fin Fill

Sand the top taper of the lower fin first.
Again, use a block.

Using a block prevents any "dipping" of the flat surface you are trying to achieve.
Using sandpaper over your fingertips could make a concave dipped area.

Here's another way to preserve the sanded taper edge.
After sanding the top, measure down 1" from the top and draw a pencil line.

Sand up to ( and a little over the line is okay) but try not to sand off the pencil line. This guarantees you'll end up with a sharp edge at the 1" mark.

The lower fin has been sanded.

Look close and you can still see a little of the pencil line from the second picture still there after sanding.