Surface on older metal building

aceypoo1

We own a facility that was built in the early 70's that is a metal building that has some type of coating on it that makes it look like a rough "stucco" or more like a "popcorn" ceiling.  Anyone have any idea what this could be made of or how it was applied?  Over the years dark black stains have appeared due to the old gutters overflowing.  We need to clean or resurface.  Any ideas would be appreciated.