Wolverine Rocketry Phoenix AIM-54A Build Part 12 Upper Fin Fill Sand

You'll need a sanding block to sand the filler down and keep the taper edges sharp.
Here I'm using 220 to knock down the bulk of the filler. I'll switch to 400 grit for final smoothing.

Don't just sand back and forth and hope for the best -
Set your block down on the outside taper. Before moving the block, be sure the flat sandpaper side is laying flat on the taper you sanded earlier.

Sand a few inches at a time, always being sure the flat side of the block is laying on the same plane.

After the long outside edge is sanded, switch to the upper taper.
Sand only one tapered side at a time - don't jump around between surfaces.

This picture shows how I'm setting the block on the same angled plane as the upper taper before sanding.

After sanding the upper taper, sand the large flat area of the fin.

If you maintained the block angle on the outside (long) and (short) top tapers, sanding the large flat area of the fin should "reinforce" and keep the edges sharp.