Frame Rail repair "cap"

Caps are just a band aid. The original frame rail will keep on rusting, and the cap will start rusting because of the trapped rust.

You had the right idea the first time. If the rust is confined to a few sections, you can cut them out and patch the rail. Just keep in mind that once you start cutting and grinding the patch will probably end up being twice as big as you think it needs to be.

If you have more than a section or two that's rusted, it's probably worth replacing the whole rail.