To dispatch darks rings, stains or gray oxidation, you'll require to applying a few ethanedioic acid crystals. It's the primary component in the so called deck brighteners. To apply them, you've to combine them with warm water till you achieve a concentrated solution meaning it won't fade away any longer. Only mix the amount you require. You'll require to use it with a synthetic brush affording the entire surface a good wet coat (not just the stain) . The wood will require being bare and gently sanded before this step. Be sure you use it to the whole surface and not just the spot. Leave the acid on till it dries out or till the stain has disappeared. Follow that with a couple washes of water to dispatch the residue. The grain will be aroused after it dries out, so you'll require sanding it smooth before ceasing. Be sure you apply a particle mask when sanding because what crystals are left over will make you sneezing if you breathe them in. Do it outdoors if possible.