Test the stain to be sure it's an iron stain. Rub a vitamin C tablet over the stain. If it disappears or lightens, the stain is there as a result of wet metal.
Place a small handful of vitamin C tablets in your plastic zip bag and seal it closed. Crush the vitamin C tablets by tapping the contents of the bag with the handle end of a table knife.
Spread the crushed vitamin C over the iron stain on your deck. Allow it to sit for a few minutes so the ascorbic acid can work on the stain.
Dampen a scrub brush and start to scrub the crushed vitamin C into the stain. Continue with this until you see the stain is gone.
Fill the bucket with warm water and rinse away the crushed vitamin C and any rust residue. Allow your deck to air dry.