I have a couple of kegs with the same problem, I use them as transfer kegs for filtering.....However I'm working at a large yoghurt making factory at the moment and have been picking the brains of the stainless guys there (and getting some choice brewery mods done too...) and on this topic I was advised to hit it with a drill mounted bristle brush (like so:
thing) to remove any corrosion, then a similarly mounted polishing wheel before applying "Pickling Paste" which re-passivates the stainless. I forget what that means...there's a appendix in the back of How to Brew that covers this sort of thing (and also a few AHB'ers who do this stuff for a living who may chip in). All in all though I served many kegs of beer out of mine before I had enough numbers to bench mine as serving kegs.
