many people spray preparations onto the underside of their vehicle.
However, rarely does anyone prepare the inside of the chassis ie the structure/frame of the vehicle.
It builds up dirt and salt (if beach driven) but the vehicle only gets a wash underneath.
Parked on a steep hill and wash out of the chassis is of great benefit if interested in trying to prevent development of rust in there. The build up of crap WILL rust the chassis through.
No good having a perfect body if the bones are eaten away.