The rubber underlay is between the paint and the Bedliner. These are best applied to paints and seam sealers, where the metal explodes with salt spray while driving in winter.
Spray some rubber primer right behind the front wheels and the floor. These things have advantages and disadvantages. You can also look for the best rubberized undercoating services via an online source.
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Like a bedsheet, the rubber spray dries to form a protective coating and, as a bedsheet, succumbs to a good rock hit or similar road debris. Because saltwater can seep behind an invisible rubber layer, salt and moisture in it can also be trapped.
It's a good idea to check these things from time to time to avoid any nasty surprises that show that the only thing left of this panel is the bearings. Applying a rubber primer can also reduce noise.
Color is the first and last armor of steel. Removing rust before it sticks to metal and applying a little paint can slow things down inevitably.
Choose your favorite combo release and power tool, chemical converter, or media blasting method, then press plain metal with paint to seal the job. I tried a metal phosphoric acid detergent from a local build which worked pretty well, then covered the metal with a crackling enamel designed to rust.