The whole bottom floorpan is new metal and there is no rust. Bedliner is waterproof whereas a rubberized undercoating will allow moisture to seep in. My truck's underside looks as good as I did it 5 years ago and I drive on the beach often and live in a high salt environment. I'm going to take the rusted portions of the firewall to bare metal and use a self etching primer then paint it matte or satin black.