A word of caution on the heavy undercoat. It holds the salt water between it and anything consumable. You can't see the damage until it's too late. When I worked for Mercedes Benz, we were notallowed to undercoat the new cars for that reason. They did, however allow us to use a product made by Bilstein that was something like cosmoline type wax. Don't know if that's even available any more with all the EPA crap.
Doug

It is still on the market. Any bodyshop jobber should have some form of BODY CAVITY WAX. We also use the same thing in an aresol to coat the inside of high end steel bicycle frames, it really works.