what they're saying is NEW metal doesn't need POR, and on old metal, surface rust makes for a better bond because then the chemical action it's based on [rust conversion] takes place, and the surface iron oxide is converted to iron tannate or some such - it's passivated, so it has some surface protection, the rest of the protection coming from it being a good primer/sealer .... they are not saying to paint over corrosion, the expanded flakey stuff, and they're not saying it won't work on clean metal .... and really clean metal won't necessarily show surface rust quickly [especially overnight], although sandblasted metal would likely soon show oxide forming in the pits if you looked at it under a magnifier
Bill
