Use or a towel or a fan to dry the area of the water damage.
Sand the wood with a hand or electric sander to remove the surface finish.
Mix a small amount of oxalic acid with 1 c. of hot water.
Brush the oxalic acid solution over the damaged area. Allow it to soak in until it is completely dry. You may have to repeat this step several times to completely remove the water stain.
Apply stain or sealer to the wood that was sanded. Wipe off any excess sealer and allow the area to dry for 24 hours before applying a finish.
Apply a coat of the same finish that was originally applied to the wood. Allow it to dry before deciding if a second coat is needed.