Originally Posted by oldhp
Cut the complete rocker off front to back. Leave it open and spray it with black truck bed liner. Done. I'll bet you can't see it anyway from the side.
You could most definitely see that area from the side if it were cut off (so a big gaping hole with rust colored metal behind it), and it is a structural member of the cab so this is not a good result. Odds are fair that the floor pan next to the area would eventually buckle without that there or if rusted too, may crack apart instead of buckling.
Your best bet is do it right, as much of it yourself as possible. If you have to pay someone to weld it, do that. Now as far as finishing and painting goes, that might not be too noticeable from a DIY rattle can job.
It all depends on how far the hidden rust has spread. There's a fair but still small chance that you just have the one area and can just replace what is there with some sheet steel if the bends aren't too complex. It may not look perfect but has to form fit very well, then you could rivet it on to hold it after joining with 3M body panel adhesive. That adhesive is essentially a special epoxy and quite pricey for what it is, though for a limited one-off use you don't necessarily need to buy the applicator, just to mix it before application, but you're going to need a lot of force to get it out of the tube, especially if it's cold.
https://www.amazon.com/3M-08115-Panel-Bonding-Adhesive/dp/B000PEW4MI
If you just want some quick and easy fix that lasts short term, grind away any loose bubbly rust, throw a piece of metal mesh behind it, slap some bondo fiberglass resin in the area, sand and paint. It wouldn't hurt to spray some fluid film (or anything greasy, really) behind that area after completed.