Well depends on your climate. In temperature swings the metal underneath the undercoating sweats. Somehow or other oxygen gets in as well. Oxidation or rusting then can be worse than on bare metal.

Petroleum products seem to block the oxygen and slow down the rusting as much as they can. You cannot absolutely stop it other than it's total elimination once underway it seems.

For some reason it is also worse in humid climates. Spraying coatings that set up over rust in general will accelerate the rusting rate. A hard coating well bonded to clean unrusted metal seems to work to some degree.

Other than that petroleum products are known to be the best alternative. Grease is better than oil. Sprayable with crude equipment if warmed up. Messy to do under chassis work afterwords unless steamed off is a problem.