So it's off the side of the cylinder around the corner from the face? If so, then the gas cutting probably initiated a peel that extends around the corner. That doesn't speak well for the adhesion.
I looked at coatings when I was working on club-owned Garands twenty years ago. The trouble spot was getting a Parkerizing matching color on the stainless gas cylinder. After several experiments I concluded that durability depended heavily on getting a sharp matte surface on the SS. Bead blasting didn't cut it; sharp Aluminum oxide blasting was required. For plain and carbon steel, a zinc phosphate adhesion layer made an even better base.
I haven't played with trying to put the current coatings on smooth steel. What I had available back then didn't hold up on any smooth surface for very long. While the newer products may do better about that, I would expect they do better still when the surface preparation I mentioned is applied.
I looked at coatings when I was working on club-owned Garands twenty years ago. The trouble spot was getting a Parkerizing matching color on the stainless gas cylinder. After several experiments I concluded that durability depended heavily on getting a sharp matte surface on the SS. Bead blasting didn't cut it; sharp Aluminum oxide blasting was required. For plain and carbon steel, a zinc phosphate adhesion layer made an even better base.
I haven't played with trying to put the current coatings on smooth steel. What I had available back then didn't hold up on any smooth surface for very long. While the newer products may do better about that, I would expect they do better still when the surface preparation I mentioned is applied.