Thanks for all the replies.
I did some price checking locally on COR-TEN vs Std A36.
11ga A36 $74.00 4’x8’ sheet
11ga COR-TEN $220.00 4’x8’ sheet
Big price difference. It is cosmetic only, so the question is long-term reliability. If i treat the A36 on the back side with a Waterproof Coating, and let the exposed side go untreated until the desired patina is achieved and seal it, what kind of life span would you predict?