A faimily friend ( Mark ) has just purchased a brand new SS Holden ute ( V8 Naturally

Mark works in the shyte ...... alot of cow shyte infact !!!! ..... He works as a concreter and is always layin concrete on farms for the cow yards, naturally this means driving down cow shyte covered driveways/tracks to get to the yards or wherever the concrete is being laid....
Now the dealer suggested that expander foam would keep the shyte out of the chassis and therefore prevent rust from getting an early start on the new ute.........
Buggar me dead but then at the father inlaws I hear the exact same recemendation, now I've heard of this idea years ago and never really gave it a second thought but the therory seems to be quite sound.
I guess my real question is should we be filling up the chassis rails on our 4x4's with expander foam ????
If it actually does what it is supposed to do then dust/mud/water would never get in and cause the rust we see in some rigs .....
any thoughts ???
Kingy