I would dip all my cars in POR-15 if I could. I have done the pan and engine
compartments of two Things and never had any significant rust through. And
you know Things, they only hold water from above. The stuff is pretty hard,
but it can be chipped. And if you don't paint over it (as soon as it
"flashes") it will turn dull (which doesn't affect its ability to keep
oxygen away from high-carbon steel). Highly recommended.