I've never had great luck filling body tube seams with Elmer's Carpenter's Wood Filler or C.W.F.
About this time last year I tried to find a good mix ratio of CWF and water.
Here's the post from last year HERE.
A good brushable, sandable mix turned out to be:
2 1/2 parts CWF to 1 part water
This mix worked well for balsa and launch lugs. But body tubes were inconsistent.
After sanding some of the filler would pop out of the body tube seams. Usually I'd end up filling these open seams in the primer stage.
I tried some different mix ratios just for body tube seams
The body tube at the right has the new, thicker mix. It was applied with a knife dipped in the filler and dragged down the seam. You do about 2 inches at a time.
Then "squeegee" off the excess with the flat side of a razor blade.
Don't cut off the filler to surface, just take off the excess.
The tube on the left is after sanding. The filler stayed in the seams!
For now, the body tube seam fill mix ratio I'm trying is:
4 parts CWF, 1 part water.