Estes Aries SST, #1927, Build, Part 9, Sanding The Rudder Edge Angle

The root edges of the rudder fins have to be sanded at an angle.

Mark the flat root edges shading with the side of a soft pencil.

The picture on the right shows the angle sand about halfway through. The pencil shading gives a better contrast so you can sand the line evenly.

I'd recommend using a sanding block, like the Estes Ultimate Sanding Bar. To reach a 3/4" dihedral height, the sanding angle is not very steep. 

On the right, you can see what's left of the pencil line. Sand until the pencil shading is gone - no further.

With a weight on one side of the rudder assembly,

Raise the other outside edge to 3/4".