Originally Posted by dogtail

WD-40 lasts and lasts, I still have last Februarys coating fully intact (my undercarriage never ever gets washed). Although I apply every snow season for 100% effectiveness. All oil type coatings use work on same principal. Proven effective for many decades by the old timers. Even stuff like Canola oil will work. With WD-40 you can also coat over rust and it will infuse in the scale and turn black and form a barrier for any further progression. I like WD-40 cause it is very thin and you can spray it on everything.
As with anything, you can half ass or not be full effective if you don't use common sense. Any coating **no matter what it is** will be broken down by repeated salting over time.....that is why seasonal or repeated applications are important.
The best method for rust prevention is the one that works for you, there is not only one formula. I spend less than 1 hour per year on rust prevention for 100% effectiveness on undercarriage and body. Here's todays closeup of the hot spot rear LCA mount. Anybody that sticks their head under my 20 yr old 370K+ mile rig is amazed at the condition for my salty region. This 4runner has been in salt country its whole life, and I drive salty roads with piece of mind I'm protected. I don't see that many 3rd gens on the road in my area anymore thanks to salt, but I'm still here solid and black.
https://www.toyota-4runner.org/3174013-post1.html