Estes Thor Agena Build Part 18 Long Adapter Fill TIP

The long transition has a an angled "step" about two-thirds the way up.
I wanted to keep the edge fairly sharp.

To help keep the edge straight, the side of a pencil tip was rubbed over the outside edge.

After brushing on FNF, the bulk of it was sanded off using 220 grit on a block.
(This self-adhesive sandpaper is closer to a 320 grit.)

Some of the pencil line sides were sanded off. But, I was able still keep some of the line and the angle edge sharper, not rounding it off.

TIP: Here's another way to clean up the joint between a adapter (or nose cone) shoulder and body tube.

Fold some 400 grit sandpaper with a very sharp crease.
Open up the joint between the balsa shoulder and body tube.
Set the creased sandpaper in the open joint.
Slide the body tube down pinching the sandpaper in the joint against the shoulder.

CAREFULLY turn the balsa shoulder against the sandpaper that is wedged into the joint.
Continue all the way around the shoulder sanding down any small irregularities.

It won't take much sanding to clean up the shoulder edge. Go lightly to define a sharp line on the shoulder.
The body tube edge will help guide the sandpaper.