Are All Undercoats Created Equal?

Take a look at fluid film. I use it on my trailer frame and truck frame. Apply it annually. The best bet is to buy the spray gun kit for your compressor and spray once a year. It is a lanolin based product and it works well...and you can use it over existing rust.
Should you not be the type that likes to crawl around on the ground, you can go to their website and find a local automotive sprayer.