OK....now I'm slowly getting to what your original idea behind that is
Like MPWR mentioned, it looks as if the entire bus was stripped and then the upper half was painted only - leaving the lower part exposed to the elements.
I suggest you should think about the following:
- build a diorama to go with it
>> this could look like: somebody polishing the white paint on the front of the car and another guy sanding the lower part of the car somewhere at the back end. you could make it look like a "poor" owner who can't afford to paint the whole car at once....but then - somebody who can afford to buy a split window bus without BAD rust (especially around the lower part) is not likely to be poor
The rust itself looks pretty good (emphasis on "pretty") - it's just too clean. Grind some holes, tears and scratches in the lower body, detail it and it'll look more convincing...
Hell...make one of the lower panels (e.g. the right rear corner) look like a new part that was just welded in...
The possibilities are endless....I'm really looking forward to what the others have to say

opcorn:

opcorn:

opcorn:
Good luck with it!