The spray recommended is Amsoil Heavy Duty Metal Protector and I have used it on my own vehicles since 2011 with amazing results.  From personal experience, the coating lasts EASILY over 1 year protecting surfaces.

From Amsoil:

Made with special rust and corrosion inhibitors. Excellent on metal surfaces, leaves a long-lasting protective coating. Protects against the effects of salt, moisture & chemical corrosion. Works as an undercoat, leaving a wax-like film.  AMSOIL Heavy Duty Metal Protector helps protect metal surfaces that are exposed to the damaging effects of salt, moisture or chemical corrosion. Ideal  for undercoating wheelwells and other metal surfaces exposed to water, dirt or road salt.

  • Helps protect against rust & corrosion, even in salt water
  • Displaces water
  • +Stops squeaks
  • +Lubricates metal surfaces
  • Leaves a dry, long-lasting, (wax-like) film
  • Lubricant does not attract dust
  • Does not sling off
  • Sprays into hard-to-reach places
  • +Also Ideal for chains & other heavy-use items