Black streaking on metal siding

Recently, I washed a lighted sign on metal siding for a business. What they wanted cleaned off was the black streaking which I was unable to remove with general cleaner. Does anyone know what product can be used to remove these streaks as well as any special technique. Thanks for your input!

Robert,
I know what you talking about, about 7 out of 10 times I've had good success with 409 and light brushing . You can buy the bigger containers at sams and then transfer that into your own spray bottle so it will look like you have something very special.
I belive its amonia based so don't mix it with bleach ..

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Shane Clardy