You could try pickling it with pickling paste. I heard that removes surface iron from SS, but don't quote me on that. I haven't had time to research it, just throwing it out there. I have one weld in a keggle that the guy used a wire brush to polish up the weld a little and my cleaning regiment helps keep it rust free, but I have to keep an eye on it between uses. Can't believe a dairy welder would use a brush that wasn't SS safe. I think that guy had it in for me. I really don't know about your situation though, maybe a couple of scrubbings with BKF Bar Keepers Friend would work to remove the rust and passivate (I think that is the word) the SS again.
Edit: Dang Sparky, you were too fast for me to promote the BKF, lol. That stuff is amazing for the inside of the kettle every now and then.
Edit: Dang Sparky, you were too fast for me to promote the BKF, lol. That stuff is amazing for the inside of the kettle every now and then.