I recommend automotive glaze. Primer base is required for best adhesion when using glaze. Use this sequence: Prime, glaze, sand, prime, sand. Old credit cards, current credit cards you probably shouldn't be using, rewards program cards, even playing cards work well to apply glaze with precision and while minimizing over-application. When sanding glaze, use a numerically ascending sequence of grits e.g. 180, 220, 320 for a smooth finish. This method also minimizes the risk of damage or "rash" to underlying glassine or other smooth surface tubing layers.
Rocket guys call it spot putty, automotive industry predominantly refers to the most superficial layer of filler as glaze, which is often used on top of primary fillers (Bondo). There are a myriad of manufacturers to choose from for either. With thin-walled, flexible cardboard tubes, avoid primary fillers (Bondo) as these will require more intensive sanding for removal and introduce greater risk of burn-through, shrinkage, staining (depending on the type of paint you choose, if using as last layer/no primer overlay), frustration and fatigue.