I'm knee deep in this issue right now, and will share a picture:
This is your jackpoint with the outer rockerpanel removed.

To renew my jackpoints, I removed the rocker panel, and cut away the rusted bottom of the jackpoint and cut a new bottom: on my car this meant I had to form a 2"dimple in the center of a piece of steel about 6" x 12". weld that to the bottom of the jackpoints extending down under the belly pan, and then replace the rocker panel.

But I have a solid set of jackpoints, and a new rocker panel. Ok, the tools to do it cost more than the estimate to replace.. but I still have the tools!

I would say that when the jack wont pick up the car without going into the rocker, the kind of undercoating is immaterial...