I know you're trying to work with what you have and I too have used rustoleum spray on my restore. But for the rear end, I used rustoleum oil enamel and a foam brush, only because the rear would seem to get exposed to the elements and the oil is tougher than the spray. I have also used zero rust, I bought it from this very knowledgeable site:
http://www.autobodystore.com/
They can tell you how to apply it.