It converts iron oxide to iron tannate so it basically creates a different coating on a rusted (oxidized) surface. You're suppose to paint over the top of it to help seal from oxygen and the elements but it's not a miracle product. It's used in marine applications all the time but it's just maintenance. It's definitely better to use it than to ignore a rusted area, but like Frank said, it doesn't replace good 'ol metal repairs, grinding, sanding, priming, painting.... 8-)
