In my case i'm removing A LOT of bad metal and welding new in it's place, i got lucky with the rocker panels, the rust doesn't extend more than an inch or two forward, i can cut it all out without any issues. BTW i'm not using POR15 or Rust Bullet or any super expensive rust treatment, i'm completely removing the rust with a media blaster, then using Rustoleum rust reformer, i've had great success with it on things like Lawn furniture that has been cleaned up and repainted, some of it even sat outside all winter without rusting again.
It's important to remember if you're going to weld in your own panels, if you are using a gasless welder CLEAN THE AREA before painting or you're asking for it to oxidize again, normal MIG welding uses a shielding gas to prevent contaminants that can accelerate the rusting process. I'm also stitch welding these panels in, ie welding in one spot then moving to another about an inch down, then coming back and filling in the gap one spot at a time to prevent the metal from warping and to ensure a snug fit, however i plan on using automotive body sealer on each patch to ensure it's water tight, I can't be too careful with my Supra
