Buy fluid film from Amazon. In 1 gallon can, black or opaque are your options. Then buy an air powered rust proofing gun that has different hoses and nozzles for cone pattern, 90 degree angle, wide angle, etc.
The fluid film is awesome, never dries out, and creeps/sticks everywhere. You will have to touch up frame areas that get blasted by water spray or pressure washer once a year, but other areas it will just stay there and keep moisture out.
Do yourself a favor and get a junk metal pan and spatula. Heat up the fluid film until it's runny like motor oil and it will spray easier and more evenly.
Done this for years on my vehicles and they will look close to new after 10 years in the WI winters. Costs a little up front for the good setup, but after that, it's probably $35/yr to do a vehicle and touch up the underside.
