Hi Brian,
That looks like a clean bus, you never know what you get when you start removing 40 years of road grime and undercoating. I am going through the same process but at the other end of the bus - the engine compartment.
What chemicals and tools are you using? I have been using USC Grease, Wax and Silicone Remover, as well as, a WalMart product called Super Clean. I use rags soaked in these chemicals and tooth brushes, needless to say it is slow and tedious. But the results have been good. Are you going through the same struggles?
Thanks, Phil.