I'm in the middle of undercoating my truck with POR-15 right now. It covers very well, a little goes a long way. You don't need to remove the rust at all, in fact it works best when the surface is rusty. You do have to remove paint though, it needs to go on the bare steel to protect it. Everyone I know that has used it loves it. One guy painted his rusty frame with it many years ago and it's still shiny black with no signs of rust anywhere (this is in western NY). It's really strong too, I coated the front of my hood before I painted it to protect it from stone chips. I really like the part about not removing all the rust, that saves a LOT of time.
Joe
Rochester Institute of Technology Team Mini-Baja
88 Jimmy