i used POR15 on the wagon, cut any rust out that was bad as i dont think there it much point trying to convert metal that is already #OOPS#ed
only used it on surface rust that could pretty much be sanded off anyway, but used the converter for peice of mind...cant really say if its worked or not, maybe get back to me in 4 or 5 years and i'll let you know

one thing to mention though, make sure you scrub the stuff off really well as it is acid, so if you leave it there the metal will just rust again. and it will do it much quicker than if there wasnt acid there