Dark spots and discoloration of the wood surface are usually caused by water spots and can be removed with oxalic acid. Oxalic acid is also used to freshen wood color if it has become grayed.

Instructions

Pour 2-4 tablespoons of oxalic acid crystals in a heat resistant glass jar and add one pint hot water. (The strength will vary according to your needs, 2 tablespoons for milder solutions and 4 tablespoons for stronger.)
Apply solution to entire surface evenly with a cloth, sponge, or brush. Do not apply just to the spot or discolored area. If you treat only the stained area, you have uneven bleaching with light spots rather than dark spots.
Let the solution remain on the wood 10-20 minutes. If the stain remains, apply more solution to the stained area only. Continue to apply until desired color is obtained.
To neutralize acid (stop bleaching action), wash the surface with a weak solution of 1 tablespoon clear (not sudsy) ammonia to 1 quart of water. If clear ammonia is not available, wash with a solution of 2 tablespoons borax dissolved in 1 pint of water. Wipe up excess moisture with absorbent cloths. Rinse with clear water and wipe with dry cloths. Remember that water will raise the grain of the wood. Let dry 24 hours before sanding. Sand down the raised grain or staining will be darker in the raised area.