A properly prepared and painted piece of steel can go for decades without the slightest bit of rust. If however you start with a piece that has started to rust and paint over the rust, it does not last more then a year or two at best before the rust is causing damage again. Why is this? The principle behind painting, as I understand it, is to separatete the iron from oxygen so that iron-oxide cannot form. If I put this paint barrier over pre-existing rust, why does it not prevent further rust formation?