Estes Saturn V #1969 Build, Part 3, Tubes Prep

The pencil line you draw down the entire length of the body tubes is the vacu-form wraps start and finish point.
TIP: Use pencil to mark the body tubes! Pen ink can bleed through the finish paint.

The instructions have you mark the locations for the rear edge of the vacu-form wraps. I'll save this step until later, the primer/filler would probably cover up any pencil lines made now.
I would mark the inside of the tube "REAR" and "FRONT" for reference.

Check the ends of the body tubes to see if there are any cut "stringers". The larger diameter the tube, the harder it is for the vendor to cut clean.

Sand the ends flat and square.

The big BT-101 tube has a lot of seams to fill. On the larger tubes, the diagonal wind angle isn't as steep.

To see the seams easier, run a sharp pencil down the seam ruts.

I did my usual  fill technique:
Dip a dull knife blade in thinned CWF.
Set the blade in the seam.
Work it back and forth leaving the filler in the seam.
Repeat down the entire tube length.

On the left is the filled seam, imagine the filler as a mushroom shape. The bead of filler overlaps onto the tube.
On the right, is the third stage tube sanded to surface. The filler is initially sanded with 220 grit. You might want to use 320 grit. The 220 I use is not a very aggressive sandpaper, it is sold on rolls to be used on my Great Planes aluminum block sander.
Follow with 400 grit to smooth things out.
Filler/primer will be done later. The filler/primer spray and sand will fill what remains of the body tube seams.

Build time so far: 
40 minutes

GOTCHA: I filled all the tubes out of habit. I realized afterwards that the third stage tube is now completely covered with a large vacu-form wrap. It's not necessary to fill and prime that tube.
Fill, sand, spray primer/filler and sand the large BT-101 and BT-58 short Service Module tube.