That dark stain on the collar of your leather coat is from sweat and body oils. The good news is that those substances are easy to remove. You do not need specialty chemicals or aggressive detergents; in fact, the less aggressive the cleaner, the better for your leather. Some household tips for leather gloves and handbags work perfectly well on grimy collars. You will need to return some moisture to the leather, be it with a moisturizing cleaner, or with a leather conditioner.
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Things You'll Need
Baby shampoo or moisturizing facial bar
Clean white cloths
Leather conditioner (with lanolin, not oil)
Dampen a cotton ball with lighter fluid. Rub lightly, not vigorously, along the stain. You should see the cotton ball become grubby as it picks up oil.
Change the cotton ball frequently, until it comes away clean. Allow the fluid to evaporate completely from one area before going over it again.
Soap the collar, using baby shampoo or a moisturizing facial bar. (Dove is particularly good on leather stains.) Mix the baby shampoo and water in a bowl, and dampen a soft white cloth; or dampen the cloth and work up a suds with the facial bar.
Rub gently along the stain with the cloth, working in circles until the stain appears removed. Do not be afraid to get the leather wet—these mild soaps will not ruin the leather—but do not soak the leather through.
Hang the coat; allow the surface to dry (up to 72 hours) to ensure the stain is gone. Repeat with soap as needed and allow the surface to dry again.
Apply a cream-based conditioner which contains lanolin to the collar. Most leather conditioners do contain lanolin and some silicone.
If you have no lighter fluid, or simply would rather not risk working with this flammable chemical, you can achieve much the same results by skipping Step 1; it will take you longer to soap away the stain, but it will work.
Take this opportunity to clean the entire coat. A once-yearly cleaning and conditioning will keep it looking new.
Avoid oil-based conditioners and neatsfoot oil, which will darken the leather.
Use white cloths, only; terrycloth if you have it. A colored cloth may transfer dye to the wet leather.
Do not smoke while using the lighter fluid, or work near heat and flame.