Rust-free Rockers?

[DenaliDan]

One requirement I had when shopping for a used truck was no rust. This made it fairly difficult to find a truck within my price range and I actually ended up going to Florida to get it. But now that I have my '02 Sierra Denali, I am determined to beat the rocker panel rot. I am even considering applying a coat of bed liner on the bottom of the rocker panel. I realize that the rust is usually caused by road salt and I plan to do my best to keep it rinsed off. What I don't know is does this rust typically come from the outside or does moisture get trapped behind the outer layer of metal and rust it out from behind. Any input and/or ideas? How have you prevented this problem?