I am Sooooo Confused over Paint

Okay, best thread I've read in a LONG time!

I've got a car that's got cancer, bad.
When pulling parts like frame rails and rocker panels and replacing them, what's the prep to insure no more cancer?

Weld through primer, weld em in, good to go, or is there a way and a product to put inside to make they stay rust free? Add drain holes?