Mix together three parts water and one part mild dish detergent in a bucket. Use a utility sponge to clean the walls of the shipping container. Wipe down each wall with the cleaning solution and allow it to dry completely.
Sand down the walls with a sanding sponge. Work in long, even strokes from the top of the wall to the bottom. Use a gentle touch when sanding, as your goal is to rough-up the surface just enough to allow paint to adhere properly.
Use a tack cloth pick up any debris left behind by the sanding process.
Apply a rust-inhibitor primer to the interior walls with a clean paint roller. Work in long, even strokes moving from the bottom of the wall toward the top. Allow the paint to dry completely and touch up if necessary.
Apply a rust-inhibitor or enamel paint to the walls with a clean paint roller to roll on the paint. Work in long, even strokes moving from the bottom of the wall toward the top. Allow the paint to dry completely.
Repeat the previous step until the desired number of coats are applied.