Most leather boots are designed to withstand a certain amount of wet surface stains, which is a normal part of wear-and-tear on boots. However, you should immediately remove diesel spills from leather boots because of the flammability of diesel and the strong odor. Leather requires special treatment. Never submerge or soak leather boots in water, and never treat a diesel stain on leather with a solvent cleaner.
Mix 1/2 cup water, 1 tbsp. salt, and 2 tbsp. baking soda in a small bowl.
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Dip a soft toothbrush into the baking soda paste, and gently scrub the surface of the leather boots, concentrating on the diesel-soiled areas. Wipe off the paste with a dry microfiber cloth.
Rinse the toothbrush thoroughly under running water. Rub the wet toothbrush on the saddle soap, creating lather. Gently scrub each boot with the saddle soap lather, adding more soap and water to the brush as needed. Leave the soap on the first boot as you scrub the second.
Wipe the boots in the same order you scrubbed them with a clean, dry, microfiber cloth to remove the soap. Allow any soap oils that remain to absorb into the leather.