I asked if it came off with a rag to understand the difference between this and “fallout” as it looks the same to me. How is it different?
This is just a wild guess but if it’s just in one spot near the door could it be the little corner where the homeowner always goes out to smoke a cigarette? Over the years it adds up?
Whatever this was, it was all along the front of the house. Usually I see it on the first 2-3 pieces of vinyl directly below the soffits. But on this house, it covered most of the front porch down to the floor.
I don’t think it’s from smoke because I’ve seen it on all sides of various houses, not just the front porch. But that’s not a bad guess.
I’m not sure what the difference is between the two. But with fallout, you use a degreaser. With this, a degreaser doesn’t work.
Get some wire wheel cleaner and try. spray on, let sit a couple of min and rinse fairly hard, I use about 600psi fan spray but only a couple of feet away
You can probably dilute it some. Stuff I use I can DS it and it works most the time. If really bad, may need to go like 2-1. Experiment and see what it takes.