It's going to be best if you get the barrel down to a consistent start point. If it's already blued, I would just Google "re-bluing a shotgun barrel" and start reading what pops up. Don't use Naval Jelly to remove existing bluing. It will work but you may screw up the job before you even start. Naval Jelly is pretty potent stuff. Sanding will work. Don't start with too heavy as that will give you some deep scratches that you may not be able to remove. I'd go with 175 or 200 to begin with and see how it works. Then work your way up to 400 or even 600 if you want. For what I've done I'm usually working with "a blank sheet" and my results have been pretty good. If you are going to "card" it and put several coats of bluing on it, make sure you wash any steel wool out with acetone so the steel wool doesn't have any oil in it.

But again, to answer your original question....a consistent "canvas" is always going to give you a better result.

Dave