i would not use any rubber undercoating just a place for moisture to get and cause rust,I would use a paint(eastwoood products have some good stuff) or bed-liner stuff like rapture bed liner and it that look like factory liner
Used Surface Sheild on mine , see how long it lasts.
After I had it done a guy told me he uses motorcycle chain wax and never has any problems with rust. He even uses it in his fishing boat that's exposed to saltwater 365.