If you have a vehicle with a unibody, live in the north where deicing salts are used, want to keep your vehicle for 6+ year, please apply an undercoating as early as possible from the purchase date to protect yourself! In Vermont anyway, a car will likely fail the required annual State Inspection, due to rust after 7 years, if you do not undercoat each year. Another revenue source for the State!
7 years might be a bit of an exaggeration, but yeah, rust remains a problem on vehicles.
Didn't Vt. experiment with reduced salt usage a few years ago? ISTR seeing signs to that effect when crossing into the state. Haven't seen them for a while...must not have worked out very well.
Our snow here in the northeast usually has a high water content, and if roads are untreated turns to ice/hardpack when you drive on it. Not sure how things are in western states where the snow is drier.
Fluid Film or equivalent is needed to minimize rust.
However I have always been totally amazed (with all due respect for those on this forum) at how people will spend $50k-$80k on a truck and then it rusts away. To me this is a scam by the manufactures and nobody seems to question it.
Actually, rust resistance is a lot better on vehicles than it was 30+ years ago. Japanese vehicles were the worst, American & European ones were better, but even so 10 years was a long time for a vehicle.
Trouble is, just when the manufacturers started to provide better rustproofing, the states switched from using rock salt to treat roads to brine, which gets in every little crevice and stays there. The plastic cladding most automakers use on rocker panels and around wheel wells not only traps it, but you also don't notice it until it's bad.
Fortunately, the town I live in uses very little road salt, but this is a winter tourist area (snowmobiling, skiing) and the state more than makes up the difference on state/federal roads.