Wash the trailer with car wash soap and water. Rinse well to remove all loose dirt and grime. Inspect the trailer and determine the areas requiring further treatment.
Mix lemon juice and coarse salt to make a paste. Cover the stained area with the paste mix and let it sit for at least an hour. Cover the paste with a wet rag to keep the paste from drying out. Wash the paste from the trailer with fresh water rinses, inspect and repeat if needed. Lemon juice and wet salt are both mild acids that can remove stains without harsher chemicals.
Mix cream of tartar and hydrogen peroxide to make a paste. Apply the paste to the stained area and keep it moist with additional peroxide as needed. Rinse, inspect and repeat as needed.
Apply a commercial oxalic acid-based rust remover to the stain, scrub with a soft brush, rinse and repeat as needed. This should be the last step if the other methods do not remove the stain. Some of the commercial rust removers contain stronger acids, and you should use them with caution. Do not leave them on the trailer surface for extended periods.
Wash the trailer a final time and apply a good paste wax regularly. Waxing the trailer will minimize future staining.
