formally being in the auto paint prepper position in a "production" repair facility I have run into undercoating numerous times. Here is the conclusive answer. Just like the pretty outside paint, it's all about that time consuming preparation, preparation, preparation. So before you "hose it on" you need to "rough up", grind out, clean, and inspect every panel that you are planning to coat. Be extra thourough in areas that are suspect for rust. Inspect the welds and seams. apply "seam sealer" in the appropriate areas. Once you are confident that any and all rust and metal repairs have been made. Then clean it. Scrub it. clean it some more. no product will stick to dirt, grease or grime that is left behind. I do not reccomend a particular brand but like most things you get what you paid for and there is usually a brand that offers good quality for a fair price. And as far as the guy above reguarding rhino liner my expirence is that it tough as nails and will not come off. So go to a true "auto paint" store and they should have a good product (bed liner or undercoating)for a fair price. Again all said and done correctly, rust and corrosion will not come back and the undercoating will protect waaaay better than doing nothing.