This reminds me of
I don’t remember what the staining was from that @Nashvillewash was washing off as the video has been gone for a while now. I’m not sure if the material you’re washing has any sort of coating/paint on it so it’s hard to give a good suggestion. If not I’d mix up some sodium hydroxide/water with a little bit of surfactant and go from there. Maybe start with 1 cup to a gallon of water and go from there. That being said, my suggestion isn’t coming from first hand experience dealing with the exact job at hand so take it as it is.
