One of the products we have had success with to clean the dark stain ( “Tannin stain” ) off of Redwood decking material is called TSP. This is a brand name. It stands for Tri-Sodium- Phosphate ( Oxalic Acid). This product is realtively inexpensive. It comes in a crystalized form. I found that, when mixed with water to the desirable dilution, I could brush it on and wash it off. The deck turns to like new condition. There are several products on the market that are quite good to clean Redwood decks. TSP seems to be and the less expensive end of the spectrum.