I used Angry Orange degreaser. It's strong enough even diluted 4:1 it melted through 2 plastic buckets. It did great on the parking lot, it would not touch whatever this mess is. In the daylight it looked like rust. At night it looked different under the lights. I tried Oxalic acid in a test area. Oxalic did an ok job on light rust. Nothing I've tried so far has touched this.
Any ideas what would work? I haven't ordered F9 yet.
Any ideas what would work? I haven't ordered F9 yet.