We have company that does rustproofing here. He does not use any type of rubberized spray or anything that gets hard at all. He developed this rustproofing material in 1973.
Is is like a heavy grease with binders and rust inhibitors.
It never hardens!!
My father had his 1974 GMC pickup done the year after he bought it. After 35 years, there is no rust.
This process is amazing. The only down side is that it makes a mess on the driveway for about 2 years in hot weather. Drips and sags, but the protection stays. It is basically self healing.
I had my 2015 Silverado done, and except for the mess when it drips, the bottom of my truck has no rust at all. The dripping stops after about a year.
Mechanics hate the stuff since it never dries and gets all over their tools.
One old time wrench did say that in 15 years, he'll be able to get all the bolts out. Just don't use it anywhere like the tongue or where you may brush against it.
It's not cheap though:
Ducarle Enterprises | Advantages of Rick Fennell Rustproofing