I bought a bunch of used Cornies and some of them have what looks to be rust on them (strange since they are supposed to be stainless). When I would remove the posts, the threaded inlets would be stained & look rusted.
I picked up some Liquid Bar Keepers Friend at the local grocery store. This stuff rocks for removing these stains. I just shaked the bottle and covered the stain with this liquid and let sit for a couple hours. Most wiped away and the rest took just a little bit of elbow grease. This stuff get's 5 stars in my book.