The problem with the brazing is the flux, it's basically an acid. Acid and paint don't mix, and in that brass that is there IS some flux, it is what is attaching it to the metal after all.
If it is ground flat, right at the very edge there is still some flux that can come out. So sealing it off good is the trick, and epoxy primer would be the best for that.
And of course on the inside if it can't be epoxy primed and seam sealed or something a cavity wax is needed to help with sealing it off.