I've only known Nat to be used externally after the cheese is formed. Possibilities here are before brining, after brining, after a few weeks of aging, or after extended aging to prevent/halt further mold development. It's either sprayed on or the cheese is dipped in a solution.
The use of these additives vary, but are more common in large operations where one ruined batch could cost 50K. AOC/DOP cheeses can't have additives, and artisan makers also tend to stay away from them.