One of the first things you’ll likely encounter is the fat over lean rule as you learn how to do oil paintings. This technique describes the structural process of oil painting, a.k.a. layering leaner paint beneath fatter paint. Following that rule lets artists create a sound construction that ensures the paint properly adheres to the canvas surface.
While there are no strict rules for approaching oil painting, renowned artists have some techniques to share with budding painters. Once you familiarize yourself with the basic principles of oil painting, you can begin creating your own way of painting the canvas.
For now, the fat over lean rule is a general guideline meant to help you develop skills and preferences and avoid unpredictable results.
What Is Lean in the Fat Over Lean Rule?
Lean refers to paint mixed with a paint thinner, such as turpentine or oil of spike lavender.
Paint squeezed right out of the bottle is simply tiny particles of pigment dispersed in a binder, either linseed oil, poppy seed oil, linseed oil, or walnut oil. Manufacturers sometimes put additives like fillers and drying agents that extend the paint and add volume.
Mixing paint with a paint thinner dilutes it and makes it wetter, runnier, and much easier to handle. It also makes the paint leaner. The paint thinner then dries by evaporation, leaving the pigment and oil binder dispersed sparsely across the canvas and oxidizing with the atmosphere.
The result would be painting with a dull appearance when it dries. It’ll also have a weaker film because the physical properties of the oil, binding and gluing, are less concentrated. It will dry faster than paint squeezed right from the tube and even quicker than paint mixed with an oil-based medium.
What Is Fat in the Fat over Lean Rule?
Fat refers to paint with high oil content, modified by adding an oily medium like linseed oil. The extra oil adds to the proportion of the binder, increasing the fluidity of the paint and making it much easier to spread. That effectively increases the paint film’s drying time, flexibility, and gloss, making it more durable.
Even when there is less pigment in the paint mixture because of the added oil medium, your dry paint film will appear much more saturated. It’ll also have more physical depth compared to paint squeezed right out of the bottle or mixed with a thinner. That is due to the glossiness of the surface increasing the strength of the colors.
Why Should You Follow the Fat over Lean Rule?
When you do oil painting, you want the previous layer to dry quickly to add another layer on top. Oil paints generally dry slowly, and it can take up to a week to become touch dry, depending on how thick and oil-rich the layer of paint is. But, the oxidation process is even longer, with some taking up to months or years to fully cure.
When the paint polymerizes and fuses with molecules to make a solid paint film, it expands then contracts as the unsaturated fatty acids connect to absorb oxygen. You want the base layers to not stress the top layers by changing dimensionally under dry paint. That way, each new layer is more flexible than the one below it.
Layering Paint with the Fat over Lean Rule
Part of layering your paint is ensuring that the previous layer has already dried. To ensure that your painting does dry from the lowest layer up, you can gradually increase your oil mixture’s oil content in every successive layer. Keep the lower layers thinner and only gradually apply thicker paint in the upper layers.
There’s no legitimate way of approaching this, except that you just need to follow general guidelines. Different artists use different methods and techniques, and having a preference will be achieved through practice.
How you approach this method, including your own creative process, is personal, so it differs from painter to painter. It will generally depend on your personal preferences and desired results.
The Different Layering Techniques Using the Fat over Lean Rule
You’ll generally use turpentine-thinned paint, neat tube paint, and medium or oil-rich paint, which is the recommended layering for oil paint. But you may also paint straight from the tube per layer.
You may also use only pigment and solvent for the first layer, called imprimatur, just enough to block the basic shapes of your painting. In your second layer, you can use paint thinned down with turpentine, then add turpentine to linseed oil in a 2:1 ratio to your paint, then topping it with a 1:1 mixture, and so forth.
You could wait for each layer to dry before painting on top of it. That means having a one-week gap between layers. This safety measure guarantees that the paint has enough time to begin with the curing process. It may not be practical when you’re time-constricted, but it’s the best way to ensure the paint dries.
You may also take additional measures to speed up drying times for oily layers, such as adding cobalt naphthenate as an accelerator. But it would also help to know that some artists don’t mind painting over wet paint. This is called alla prima. Moreover, some artists also prefer using paint straight from the tube and increasing the upper layers’ oil content.
What Happens When You Don’t Do the Fat over Lean Rule?
If you’re painting over an oily layer using lean, inflexible paint, you’ll likely encounter structural problems with your painting, which may affect the appearance and permanency of your piece. Once your paint becomes touch dry, layering lean over fat will look chalky. The glossy and saturated layers underneath might also look as if there is a matted film over it.
Once the lean paint reaches the end of its curing process before the oily layers below, it could shift and crack due to being bound to a moving surface. Parts of the paint may also come off the surface.
Being aware of the hierarchy of drying speeds and how solvent versus oil affects this puts you at less risk of encountering the problems detailed above.
The Fat over Lean rule will help you achieve better results in oil painting, especially if you’re just a beginner. Learning the basics of oil painting and the fundamentals of layering recognized by many generations of painters helps you hone your skills and maybe create your own method. But for now, the fat over lean rule will guide your way across the canvas.
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