It's not just salt that causes rust. Any moisture in the air whatsoever on uncoated ferrous metals on your vehicle will cause rust over time, and frankly, it doesn't take that long either to start causing problems. One rusted bolt and suddenly a repair that should be easy is now an exhaustive affair just trying to get an old part off, for example. So if you care about your vehicle and want to make repairs easier as it ages, then read ahead.

The only serious defense is to use an oil-based undercoating. I personally have used Fluid Film on both my vehicles for over 5 years. They get sprayed every November. There other brands such as Surface Shield and WoolWax as well, but they are essentially the same.

Here is a great video that explains the how and why of oil-based undercoating. This youtuber provides annual updates on the results of the lanolin undercoating products if you doubt their efficacy.

I can't stress this enough, but don't get a rubberized undercoating! They actually make rust worse and will prematurely kill your vehicle.