Have you thought about using automotive-grade paint on the body instead of Rustoleum?
There are a ton of places one the internet that will sell a gallon kit for less than $100. TCPGlobal sells a gallon kit for around $75. Here's a link:
http://www.tcpglobal.com/restorationshop/rspcolors.aspx
I know that's about four times the cost of Rustoleum, but it's still pretty cheap and the results will be at least four times better. Plus, you'll get the experience of shooting real paint, which is what I assume you want to do with your 1/2-ton.
Just my two cents...
Ed