Hi, Joe. I assume that the problem is a black water stain. If it is, the first step is to remove the finish on the whole piece. If you have an oil finish, this will be very difficult to repair. The patina that has developed over the years can never be totally brought back after you have removed the stain and the water that will be used will raise the grain. While it is more difficult to remove a film finish--varnish or lacquer--it is easier to remove the stain and blend it in with a film finish. But putting a film finish back on the piece will still be difficult. Because of the natural oil in teak, a varnish based finish will take a long time to dry and a lacquer or water based poly will not bond well with the wood. If this is an important or expensive piece, I suggest that you have it done by a pro.