A cut piece of square stock on ETI 9843 at Crash Course Heavy on September 30 2017.
After the firemen cut the structure, we get a chance to look inside vertical pieces of metal, and what do we find? We don't find fully rusted thru parts.
One conclusion that may be drawn is that due to the heat/cool cycles the buses see (they are garaged each night), water is condensing inside the square stock. And it has nowhere to go but down.
The lower portions of the structure are closed, and thus the water pools and rusts the structure out from the inside. One can rustproof the outside all they like.
It won't make a difference on the inside.
rtm photo