Better late than never...I had mentioned I use a rustproofing product in a post re rocker panel rust, and in reviewing the post I think you asked what the product was. It's from a company called RustCheck, and I buy it at Canadian Tire- no idea if you can get it in the US. Comes in aerosol, one type is thinner consistency for creeping into tight spaces, the other is thick and gooey-great for exposed areas like the underside.
cheers