Almost all of us have had this happen to our tables. A water mark left behind by one of many culprits. It could have been glass of water, a hot casserole dish, plant, or a pizza box, leaving behind a nasty condensation stain that looks more like a ghost invaded the interior of your wood. Here is a trick I have used, one too many times.
Fold a DRY cotton cloth or towel in half (thirds if very thin) and then use an iron, WITHOUT using the steam setting, on top of the towel.
Move the iron back and forth constantly while keeping the cloth still. (I even read some people have used paper towels, but I have not tried that myself). Do not stop in one place and make sure to stay on the towel. The heat of the iron, going through the towel, should make the water in your wood rise to the surface.
Lift and check the surface once you know the heat has gone through the cloth to the table. The towel should now be wicking up the water mark.
I have used this many times, but beware... it could make the stain worse or damage your table if not done properly.
Other DIY remedies to remove water stains from furniture or tables.
Many people have used toothpaste, baking soda, mayonnaise and cigarette ashes.
If you want to try toothpaste, select the white, not the gel kind, and rub it into the water stain using a soft, damp cloth. Wipe it off with a dry cloth when you’re done and finish with regular furniture polish.
Some opt for baking soda which should be blended with some water to turn it into a paste (1 tablespoon of baking soda to 1 teaspoon of water should do the trick) and leave it on the stain for approximately 30 minutes.
I am not sure about this one, since I have tried the others, but if you use mayonnaise, it should be left on overnight.
Cigarette or fine fireplace ashes should be mixed with water and gently rubbed into the stain, and as with the toothpaste, all treatments should be removed with a dry cloth and polish.