If you can see it from the ground I would not roll the primer onto the bus. I say this because your going to see the blemishes right through the color coats. If it is in an area where you can't see, like on top, then I would use the elastimer coating to help insulate and cover the roof. Elastimer is about $75 for a 5 gallon bucket and you can roll it on. I did 3 coats and it is over a 1/4 inch thick.If your going to be able to see it then I would spray the primer, sand again and spray your color coats.
3 coats with about a gallon left over.
I am using 3M paint stripping disks, wire cups on an angle grinder, and then going over the surface with a DA with 180 grit and using stripper around the rivets. Not trying to get all the old paint and primer off but smoothing and blending the surface into the bare areas. Any one see anything wrong with the plan or have any advice???