Estes Super Orbital Transport Build, Kit #7314, Part 17, Joining The Tubes & Filling The Seam

The two BT-55 tubes are joined by a red coupler.

Before gluing - 

TIP: Do a dry test fit. Glue the red coupler into the lower tube. Slide on the upper tube (dry, no glue) and turn the upper tube to find the position of the best fit, making sure the tubes are in line. Mark this position with a pencil line over both tubes. Let the lower half coupler glue dry. Apply glue inside the upper tube. Slide on the upper tube and rotate to align the best fit pencil marks.

While the glue is still wet, 

Use your burnisher to mold and even up the joint edges. Let dry.

The tube seam is filled much like filling balsa.

First use CWF to do the bulk of the filling.

After sanding the CWF, you may have roughed up the tube.

A wipe of medium CA glue will harden up the edges for smooth sanding. 

Next up is a shot of filler/primer to fill in any remaining joint seam.

The launch lug and tip of of the forward wing strake is taped off from any filler/primer overspray.

Here's the seam after filling and sanding.

It will be barely noticeable after the gloss white is sprayed.