Sometimes you can use Chlorine Bleach. Chlorine will usually remove some stains, particularly dye stains that Oxalic Acid won't. But
chlorine won't penetrate very deeply so if you do much sanding you may sand down to the stain again. Oxalic Acid will usually remove water, alkali and rust stains. Two-Part Wood Bleach will also remove these stains
but it also removes the natural color from the wood. Both Oxalic Acid and Two-Part Wood Bleach should be available at most paint stores. The drawback with all these products is that they have to be applied to the
entire surface. You can end up with a table, for instance, that has nice brown walnut legs and skirts but a VERY light top. You must also be very careful to remove/rinse off any residue or there could be adverse
reactions to any stain/finish you apply. Be sure to follow directions accurately and fully, both to protect the piece and yourself. It is usually a good idea to do a test on a very small area, an out of
the way spot or even on some scrap just to get the feel of the product and see just how much color gets removed.