Slip on a pair of rubber gloves and create a mixture of one-half gallon warm water and one-quarter cup oxygen bleach powder. Stir the ingredients until the powdered oxygen bleach is completely dissolved.
Dampen a plastic scrub brush with the mixture and gently scrub the affected sandstone step to remove any dirt, grime and grease. Dampen a soft cloth with water and wipe down the sandstone to eliminate the oxygen bleach residue. Dry the stair with a separate soft cloth.
Dampen a chunk of #0000 steel wool with plain water. Gently rub the affected chunk of sandstone with the steel wool until the scratch is eliminated.
Apply a thin layer of sandstone or natural stone sealant to the stair with a lambswool applicator. Allow the first layer of sealant to dry according to the package directions before applying a second thin coat with the lambswool applicator.
Allow the final coat of sandstone sealant to cure for at least 24 hours before touching or cleaning the sandstone stair.