It was more a suggestion with regards to preventative maintenance really.
If it helps, my own routine for dealing with rust is first to do whatever is necessary to get back to bright clean metal, then application of any of the proprietary rust treatments (e.g. Bilt Hamber), light key, etch prime, light key, fill/shape, light key, glaze, light key, 2k Lechler direct gloss.
For a lap joint such as the roof seam it’s more about getting the rust out. Judicious use of a wire wheel might help, a bodyshop type Roloc or Mule Skinner wheel (wire in resin). Then treat, prime, paint and seal as best you can.
I did see some pretty bad cases of roof joint rot on YouTube involving slit wheel grinders and fibreglass, but they were well advanced examples.
Also the theory that it starts from the inside is just my own thought, based on the observation that rust was first visible inside the roof of my old T1N when the external joints appeared sound. I think 906s (NCV3s) have superior external sealing, but the internal joints are just as exposed as the 903s / T1Ns.