How do I get a greasy mark off a leather sofa arm?

Why the usual answer works

Body oil on an armrest or a headrest is ordinary greasy soil, and the two things that lift greasy soil are a surfactant and a solvent. Washing-up liquid emulsifies the oil so water can carry it; alcohol dissolves it outright. Both are in every kitchen, both are what people reach for, and on almost any other surface in this catalogue one of them would be the right answer.

Usually recommended: Detergent, Isopropyl alcohol.

Why it must not be used here

The leather trade body prohibits both by name, and the reason is the finish rather than the hide. Modern leather finishes are WATER-BASED, and its banned list is built around what attacks them: any high pH cleaner, cleaners containing alcohol, strong solvents, and caustic household cleaners such as soap or dish detergent. The stated consequence is cracking or other damage to the surface. It is the same reason saddle soap and mink oil are on the list beside glass cleaner - traditional leather remedies formulated for finishes no longer used.

This damage cannot be undone by cleaning. There is nothing left on the surface to remove.

More about finished leather and oil and grease on textile.

What to do instead

Take the dry route first, which is the trade body's own procedure and reverses the direction the oil travelled. Place CORNSTARCH over the mark and rub it gently with a finger until it coagulates with the oil, repeating as necessary. Only when the absorbent has taken what it can does anything wet follow, and then it is a cloth dampened with odourless mineral spirits rather than a household cleaner. Technique carries as much of this as product. Cleaner goes onto a clean white cotton rag and NEVER onto the leather or into a spray. Work in a circular motion under moderate pressure, and the guide gives a memorable ceiling for that pressure: no more force than a firm handshake, because scrubbing damages the leather and makes the stain harder to remove. Air dry, with no heat. Two further limits are worth knowing before starting. Deionised water is specified where water is used at all, because minerals in hard water leave rings.

• Absorbent powder treatment — Lifting an oily mark out of a porous surface with a powder rather than dissolving it with a liquid. The gentlest route on anything water would ruin.
• Solvent wiping — Dissolving an oily soil and letting the solvent evaporate. The answer where water would leave a mark or cause harm.
• Finished leather — Modern leather carries a water-based finish, which is why the traditional leather products — saddle soap, mink oil, wax — are on the prohibited list rather than the recommended one.

This is a case where a specialist is a reasonable answer. Restoring a surface is a different trade from cleaning it, and continuing to try cleaning products on damage generally makes the eventual restoration harder.

The chemistry behind both halves

• Solvents and polymers — A solvent cannot tell a soil from a surface made of similar stuff. Most modern surfaces are polymers, which is what makes this the commonest way to ruin one.
• Capillary absorption — Liquid moves through a porous material on its own, and it moves outward. It is why a stain grows after you treat it, and why a powder can pull one back out.
• Material compatibility — The reason CleaningHQ answers questions about surfaces rather than about products. The same agent is the right answer and the wrong answer depending on what it lands on.