If your leather has developed a whitish coating, you might be forgiven for assuming that it’s mould. In fact, it could well be a fat mark. 

This is caused when residual fat from the leather rises to the surface, usually as a result of fluctuations in temperature and humidity. At first glance it looks like white mould.

The causes of fat or grease marks on the leather surface are different and affect several steps in the tanning process. Inadequate degreasing of natural fats in the skin, as well as the oils and fats used for rehydration can promote fat rashes.

These grease marks are no real cause for concern. They emerge almost exclusively in leather clothing and leather bags. Car interiors, shoes and upholstery leathers are rarely affected.

Unfortunately for the tanner, this phenomenon does not arise immediately after the production of the leather, but weeks later. Most of it happens whilst still inside the transporting container or when the leather items are packed in plastic. But changes in humidity play a big role, especially once the fat has emerged, and it will reoccur after cleaning.

Unlike with mould, heating can make fat temporarily invisible as shown on the image below.

Melted fat on leather surface

Where fat marks can occur

Firstly, fat marks are common in new material, which was manufactured in Asia and packed into transport containers. Goods packaged in plastics in very humid places cause the tanning to sweat out during delivery. 

Fat marks can also appear on old leathers treated with waxy care products. Excess sweating can occur when temperatures or humidity rise. As mentioned, fatty stains can be mistaken for mould. To check, simply run a hair dryer over the affected area. Wax or grease will temporarily melt and disappear, whereas mould will stay the same. 

You can keep light fat marks under control with the hairdryer method. Fat reacts to heat so, after just one application, the marks will be gone.

                           Michael Kors leather belt with fat marks     Michael Kors leather belt cleaned

For medium fat stains on water-resistant leathers, the following procedure is recommended: clean the leather with a mixture 1:10 of Colourlock Leather & Fur Wash Concentrate and water. Apply thin layers of the solution using a wet cloth and let dry on its own or use a hair dryer to speed up the process. Protect the surface with the Colourlock Leder Fein Care & Waterproofing Oil. Although this cannot guarantee that the problem will not re-occur, this method is much faster than the constant blow drying and wiping. More solid fat or wax which causes the stain is diluted gradually and loses its effect.

Professional cleaning with solvents will be required in order to completely degrease the leather to make the fat rashes disappear permanently. The same applies to porous leather and water-sensitive smooth leathers. Professional cleaning is often a good idea for large quantities (like container goods).

                            Leather car seats with fat marks     Leather sofa with fat marks

The science bit

This test has been developed by Gerber Anderas Ollert. Even if he was not scientifically verified, his testing work has always been helpful.

Add 50-100 ml of tap water in a 500ml lab beaker with a large opening. Cut a piece of leather to be tested so it fits the beaker. Fix the leather with a rubber band over the opening of the beaker. The leather piece should also fit into the container. Place the leather with the grain side upwards and close the beaker with it. Heat the water and cook for 2 minutes or longer if the leather is thick. Steam should emerge from the visible scars.

This should shrink the leather. Dry the beaker entirely. Place the leather piece inside the container and make it airtight with plastic wrap and a rubber band. Leave for 3-5 days at temperatures from 20-25°C. If the leather hasn’t developed any fat marks during the test, then no fat rash is to be expected by the end customer.

Top Tips!

  • Use a hair dryer to melt the fat and wipe it away

Recommended products

For water resistant leathers :