Estes Maxi-Brute Honest John (Kit #1269), Part 13, Thinning Down The Fin Sides

You are instructed to thin out the fin edge overhang to 1/2 of the original thickness. This is much like the old two-sided vacu-form fins in the older Centuri style Saturn 1B and Saturn V kits.

   

The big fin plastic halves are flexible.

Don't sand like the picture on the right - 

Sand as shown on the left, flat over two sides.

This prevents any scratching (and possible  thinning) of the inside wall.

After some scratching up the interior of the first fin half,

I taped the inside wall to protect the surface. 

Notice the forward fin half. The side edges have been thinned.

Note the inside corners are sharper, more defined than the unsanded fin in the back. 

I ran a Sharpie marker over the edges to show the thickness.

On the left is a fin half, not yet sanded and thinned out.

On the right is a sanded, thinned fin edge.