don't even waste your time, regular ole spray paint won't last no matter what you do.
they make undercoating spray.
i found a product i love and recomend to everyone for this sort of thing, i did my whole frame on my build. it is crazy tough stuff, and even with just a little prep, this stuff will last a long time. its called "rust shield" and the company is sem. look into it, if you wanna spend just under 100 bux total, and never have to do it again. if you want more info on it, let me know.