Originally Posted by Gr8wht2000GMC
really rednecky but it works well.........
fill rocker with expanding foam, when cured, trim off excess with steak knife, dig/drill into foam to anchor fiber filler, apply long strand filler being sure to get it into anchor areas, sand and paint with your color. I (shamefully) did this on an 89 crew cab with really long rockers and it worked/looked good for about 4 years before it started breaking up.
That foam retains a TON of water. When I had to redo my boat, and was tearing off the old floor, the pevious owner filled many of the cracks with that great stuff. Not sure I'd want all that soggy foam in there.
If you cant get someone to weld new panels in, you could always prep the old spots with rust transforming paint to seal it all in as much as you can (if you dont want to cut it out) and then rivet replacement steel in place and bedliner it or something.