Cleaning leather involves not only the removal of the offending stain or substance but the preservation of the grain and texture as well. While some soiling will come off leather with saddle soap or treatment oils, some stubborn stains simply remain. This is when acetone is worth a try. It cleans the soiling and also preserves the integrity of the leather.
Brush the leather with a soft brush or damp sponge to remove any debris. Allow the spot to dry if you've used a damp sponge, so you can actually determine the extent of the soil or stain.
Soak the cotton ball in acetone and squeeze to remove the excess liquid. Dab the cotton ball onto the soiled area for a minute or two. The acetone will help to lift the stain but will not harm or change the color or texture of the leather.
Allow the spot to dry for about half an hour. Determine if the stain is gone or lessened. If it is lessened, repeat the process with the cotton ball and the acetone.
Apply a leather conditioner to the leather once the acetone has dried. Allow the conditioner to soak into the leather, and buff with a soft cloth per the manufacturer's directions.