There are many types of stainless steel. The kind involved in this instance might not have been truly weldable in the sense of no change in quality happening during that step, which means only welded seams got affected chemically. Or perhaps there was a do-it-yourself repair error involving another metal. Just rough it up a bit it down with a brown Scotchbrite pad and apply marine expoxy putty or moisture-cure marine caulking (7 to 10 dollars a tube - don't be fooled by something cheap). Metallic-color Silicone is OK but might need to be replaced after a few years. The so-called stainless sinks sold at home centers will sometimes rust within twenty minutes where ketchup or lemon juice is present. True lab-grade stainless for sinks is typically 316L.