Short answer: on vegetable-tanned leather, oiling the rough side (the flesh side) is common and generally fine, applied in thin coats. On suede, nubuck and roughout, the rough side is the surface you want to keep, so the default is not to oil it.
What “the rough side” means
Leather has two faces. The grain side is the smooth outer surface. The flesh side is the rough inner side of the hide. On suede, nubuck and roughout leather, the rough side is the visible surface, so the same question means something different. Check which kind of leather you have before you reach for the oil.
Oiling the flesh side of vegetable-tanned leather
Oiling the flesh side is common practice on vegetable-tanned leather. The flesh side is more open, so oil penetrates it more readily than the grain. Saddle makers have long oiled tack and saddle skirts from the flesh side for deep penetration. Oiling from the flesh side also darkens the grain less, and less unevenly, than oiling the grain directly.
Oil will still darken the grain somewhat as it wicks through, so test on a scrap first.
Choose an oil
- Pure neatsfoot oil is preferred by many makers.
- Compound neatsfoot oil contains petroleum oils, so many makers prefer the pure version.
- Mink oil and specialist leather conditioners are other options.
Avoid cooking oils. They can go rancid and darken leather.
Apply thin coats
- Start with clean, dry leather at room temperature.
- Warm the oil slightly. Warm oil and room-temperature leather absorb more evenly.
- Apply a thin coat with a cloth, sponge or brush.
- Let it absorb, often for several hours to overnight.
- Wipe off any residue, then decide whether another light coat is needed.
Light, repeated coats work better than one heavy soak. For how oil and sunlight change the color, see how to darken leather.
Lined goods: condition the grain instead
Most bags and shoes are lined, so the flesh side is out of reach. For these, condition the grain side with a conditioner instead. Many items only need occasional care on the grain. For cleaning and conditioning the grain, see DIY leather shine.
Suede, nubuck and roughout
Do not oil suede or nubuck. Oil mats the nap and darkens it permanently.
Roughout boots and saddles are a more debated case. Some owners apply a very light oil and accept that it darkens and flattens the nap. Others prefer to clean and protect roughout with a hard-bristled brush, a suede eraser or a neutral spray conditioner, which keeps the color and nap unchanged.
Our judgment: start with brushing and cleaning. If you do try oil, use a very light amount on a hidden patch first, and accept that the color will change.
Signs of too much oil
Over-oiling makes leather limp, stretchy and greasy. It can also leave a whitish, fatty bloom on the surface, sometimes called spew. If that happens, wipe off the excess, let the leather dry, and hold off on more oil until it recovers.
Common misconceptions
- Oiling the flesh side does not waterproof leather. Oil conditions the leather. It does not seal it. For what does help, see is vegetable-tanned leather waterproof?
- More oil is not better. Light, repeated coats beat one heavy soak.
- The flesh side does not need oil on every project. Many items need only occasional grain-side care.
- Cooking oils are not suitable. They can go rancid.
For the wider finishing sequence, see leather finishing.