Properly seasoned wood is thirsty and more than ready to absorb whatever water you leave standing on it, which is bound to leave a stain. If the water contains iron or other minerals, the wood can react with the minerals and it may even turn black. Most furniture has a protective finish to prevent this, but liquids can seep into the finish and discolor it. Handling wood and finish stains calls for different procedures.
Wood finishes are water resistant -- which is one of the main reasons for using them -- but few are waterproof. Standing water, steam and even persistently high humidity can turn many finishes cloudy -- a type of discoloration caused by moisture seeping into the finish. When water seeps all the way through the finish and penetrates the wood, a different kind of stain results. Some wood species, such as fir, may simply turn dark, but other species with a high tannin content -- primarily oak -- can turn black, as if they were moldy. Wet oak often is moldy, and the mold contributes to the black discoloration caused by tannin reactions with minerals in the water.
Cloudy or milky water stains that affect only the finish aren't that difficult to remove from furniture, countertops or floors. The basic principle is to remove the moisture or replace it with an oil that won't turn cloudy. You may have luck simply heating the wood with a hair dryer to turn the water to steam and boil it away. You can also cover the stain with an oily substance, such as lemon oil, petroleum jelly or even mayonnaise. Let the oils seep into the overnight, and when you wipe the wood in the morning with a dry cloth, the stains should be gone.
Stains that have penetrated the finish and soaked into the wood can't be repaired unless you remove the finish. To do this as efficiently as possible, sand the finish by hand with 120-grit sandpaper, going with the grain of the wood and feathering the edges of the area you sand to make the finish easier to repair. Once you get to the bare wood, you may be able to remove discoloration by [scrubbing with soap and water to remove mold](http://www.epa.gov/mold/cleanupguidelines.html). If the wood is still dark, you'll need to bleach it. The most effective bleach for water-stained wood is oxalic acid; you can buy pre-mixed wood bleach, but you can make a stronger compound by mixing it yourself.
A saturated solution of oxalic crystals and water gives the maximum bleaching power-- a saturated solution is one in which no more crystals will dissolve.
• Put on rubber gloves and a face mask. • Paint the solution on the bare wood. • Wait for the crystals to dry, then vacuum them. • Repeat the process if the stain isn't completely gone. • Mix a tablespoon of baking soda in a cup of water and wipe it on the wood to neutralize the acid when you've finished bleaching. • Rinse with clear water and let the wood dry. • Repair the finish by brushing or spraying two or three coats of new finish.