POR15 is great. i've used it hundreds of times. Best way to handle rust is to sand it off or blast it off but thats not always an option. POR15 I have used on frames, undersides, inside of vehicles, etc and the frames hold up great. I've used it on customers cars and years later it still looks great and no rust problems. Key is that a little goes a long ways so apply it thin and only apply 2 or 3 coats. If applied thick it will peel when dry. Dont try to cover it completely with one coat.