Fat over lean is the one rule in oil painting that decides whether a finished canvas stays sound or cracks a year later. Angela Bandurka shows it on a plain white canvas so you can see every layer: a thin first wash, a paint-only middle layer, thick top layers and loose detail work. It all comes down to one question. How much oil or medium is in each layer?
What fat over lean means
"Lean" paint has little oil in it. It is thinned, dries fast and sits flat. "Fat" paint has more oil, so it stays flexible and dries slower. Each layer you add should be a little fatter than the one under it. Angela also calls it thick over thin, which is easier to remember.
Get it backwards and a fast-drying lean layer ends up on top of a slow, flexible fat one. The bottom keeps moving after the top has set, and the top cracks. That damage can take months to show.
How to thin oil paint: your options
| Thinner | What it does | Use it for |
|---|
| Fast-dry alkyd medium | Thins and speeds drying | The first lean layer, about 1 part medium to 5 parts paint |
| Odorless mineral spirits | Thins with no added oil | Washes and toning, in a ventilated room |
| Walnut or linseed oil | Adds fat and flow | Late detail work only |
| Nothing | Paint straight from the tube | Middle and top layers |
Angela keeps medium to no more than 20 to 25 percent of the paint. More than that and the film gets weak and glossy in patches.
Painting solvent free
You can skip mineral spirits completely. Use a walnut alkyd medium to thin, and clean brushes in a jar of walnut oil wiped out on shop towels. The jar of oil darkens over time but keeps working for months. Wear gloves either way. Oil paint pigments are not something to rub into your skin.
Darks thin, lights thick
Most oil painters block in their darkest values first, thin and lean. Each pass gets lighter and thicker, and the brightest whites go on last, often with a palette knife. You can still go back and forth to fix an area. It is a rule of thumb, not a law.
Drying times between layers
Painting alla prima, wet into wet, you can build every layer in one sitting as Angela does here. If you tone the canvas with a thin oil wash first, give it 24 to 48 hours before you paint over it. An acrylic wash dries in minutes, so it is the quicker choice for toning.
Common mistakes
- Starting too thick. A heavy first layer leaves nowhere to go, and the next layer smears into it.
- Adding oil early. Save oil for the end, where it helps fine lines flow.
- Too much medium. Keep it to a small share of the paint.
- Muddy color on top. Wipe the wet paint out before painting a light color into it.
Keep painting
Get the surface ready with how to stretch a canvas and how to prime a canvas. Put the layers to work in an oil landscape, and build thick top layers with palette knife painting. Working in acrylics instead? See how to thin acrylic paint.
