I am looking for something definitely NOT high tech. I sure as hell can't afford $2K for the underside of the car. As I scraped and cleaned the undercarriage, I would sandblast a 3'x3' patch, clean it up and spray some primer on it to prevent flash rusting. Then, the next time I had time (and the weather was right), I would do another patch. So, I've ended up with a literal patchwork of rattle can primer on the bottom of the car. I really don't want to have to do any additional surface prep before applying some kind of undercoat and color. The car had a layer of tar-looking undercoat when I got it; it was absolute hell to get it all off so I could inspect and/or repair.
