We use "Wood Bleach" it is Oxilic acid and come as a dry power you mix with water in a bucket then apply with your deck brush. No need to scrub....apply let sit for 5 minutes the wash off and the stain is gone. The little container below makes many many gallons. Mix it weak at first then more strong until it removes the stain. Works great on rust too...and is cheap and widely available at Ace hardware and paint stores.