Sealing the Tubes
Perhaps the most important contributor to prolonging the life of your cardboard rocket is sealing the ends of the tubes with thin CA. I've "enjoyed" enough water landings to believe there's sufficient empirical evidence that sealing the ends helps prevent lasting damage to paper airframes. To me, it's a given and a no brainer, so here we go!
...one pass or layer on the left, none on the right. One can see the difference as you go...
...I like to make 2 passes, the first fully soaks in as on the left side of the following tube, the 2nd soaks in just a tad, leaves a bit of a sheen to the surface, and tends to run onto surfaces on the inside and/or outside of the tube, which is quickly wiped up with a handy paper towel.
After the CA dries for a few minutes (and your eyes stop watering, and your nose stops burning), the ends of the tubes are sanded with a sanding block to take down any high points.
Boom, the tubes are ready for primer!