I'm with John, I have been trying to stop cars going rusty for 65 years now! The most successful treatment so far has been spraying the underside with oil once a year, paying particular attention to the cavities.
The sills I treat by squirting oil in through any orifice I could find, if there isn't one, removing one of the tread plate screws (if it has them) is effective, you need to keep squirting it in until it starts to run out.
I have used Waxoil but unless the weather is very hot it tends to set before it penetrates the seams which is where the rust often starts, oil on the other hand will and keeps on creeping. Waxoil is very good for exposed surfaces all the while it stays there and for making sure nuts and bolts come undone next time you need them too.
Grease is also an excellent rust retardant all the while it stays there, you need to remove any loose mud then plaster the backs of the panels with it but it will need doing regularly but lasts longer than oil. Its also good in cavities but not as easy to get in as oil.
Peter