Hi, Sandy. Probably you could have saved yourself some work and saved the patina on your wood if you had tried bleaching first. Get some oxalic acid based wood bleach from your home center or specialty paint store and give that a try. Mix it with water and apply to the stained area. If you can do this out in the sunshine, so much the better. Then rinse with plain water, let dry a little, and then rinse again with water with some household ammonia in it to neutralize any remaining acid. If it is a water stain, this should get it out.
On the chance that it is not a water stain and the oxalic acid does not work, go thru the same process replacing the acid with household bleach and vinegar for the neutralizer.
Be very careful with these chemicals, particularly the oxalic acid. Follow the manufactures directions exact and use chemical resistant gloves and eye protection. Add a dust mask when mixing the acid.
Putting water on freshly sanded wood will raise the grain (cause little wood fibers to stand on end). Remove these with a light sanding with about 150 grit paper, then tack rag and proceed with the finish of your choice.
This should solve the problem. Good luck. Henry in MI