Well, my experience of undercoating has been that unless VERY thoroughly applied it does little good. Most applications are spotty and incomplete leaving all-important areas unprotected. The joints between panels, the insides of doors, the various small places where water with salts can enter but thick undercoating cannot, and several other places that are vulnerable make undercoating unreliable.
Better to be sure to spray the entire underside of the vehicle with a known rust preventative and penetrating product like POR-15 (name?) and do everything possible to keep the undersides clean after use. Painting available parts can help too.
Rust is pervasive stuff - once it starts it's hard to control and it usually starts where it could never be touched by heavy undercoating products-- They offer false security at best.
JMHO!!
(edited for spelling)