‘Fat over lean’ is one of the first rules you will encounter when you learn how to oil paint.
It is a technique that describes the structural process of oil painting. ‘Leaner’ paint is layered beneath ‘fatter’ paint. When followed, artists create a sound construction to ensure the paint will properly adhere to the surface.
There’s no real ‘how to’ when it comes to oil painting. As soon as you’re familiar with the basic principles, you can start finding a way of painting that suits you. The fat over lean rule is a general guideline followed to diminish unpredictable results.
I’ll describe what the different terms mean, why the rule applies to oil painting and different measures you could take to follow it, so that you get the best results from your painting.
What is ‘lean paint’?
Lean paint is paint that has been mixed with a paint thinner such as turpentine.
Straight from the bottle, paint is simply tiny particles of pigment dispersed in a binder. Binder is usually either linseed oil, safflower, poppyseed or walnut oil. Some manufacturers add additives such as fillers and drying agents to extend the paint and add volume.
When mixed with a paint thinner, or solvent as it’s otherwise called, it dilutes the paint and makes it much wetter, runnier and easier to handle. It also makes the paint leaner.
Solvent dries by evaporation. This leaves the pigment and oil binder dispersed more sparsely across the canvas to oxidise with the atmosphere. For this reason, when the paint is touch dry, it takes on a much duller appearance. The paint has been diluted, is drier, and it will have a weaker film. This is because the physical properties of the oil molecules (polymers), such as the binding and gluing properties are less concentrated, having being spread more thinly across the painting surface.
It dries much more rapidly than paint from the tube, and even more quickly than paint that has been mixed with an oil-based medium….
What is ‘fat paint’?
Fat paint is paint that has a high oil content. This means it could have been modified by adding an oily medium such as linseed oil. Adding extra oil is just like adding to the proportion of binder in your paint.
Using oil as a medium, in terms of its handling properties, will increase the fluidity of the paint, making it easier to spread. It will increase the drying time, flexibility and gloss of the paint film, making it much more durable.
Although there is less pigment in your paint mixture when you add extra oil medium as it is more dispersed throughout, your dry paint film will likely appear much more saturated or have qualities of depth when compared to paint straight out of the bottle or paint mixed with solvent. This is due to the glossiness of the surface increasing the strength of the colours.
Different oil media have different properties, as not all create what many artists would describe as a ‘strong’ paint film. To learn more about this, consult this post about different oil painting mediums.
Why follow this rule?
When using oil as a medium, you want the previous layer to dry quicker than the one you add on top.
Oil paints can take up to a week to become ‘touch dry’ depending on how thick and oil-rich the layer of paint is. However, it can take months, even years to fully cure and undergo the oxidisation process.
When the paint is polymerising and essentially fusing with molecules to create a solid paint film, it will expand then contract as the unsaturated fatty acids link up to absorb oxygen. You want to ensure that the base layers of your painting do not impose stress on the top layers by changing dimensionally under paint that is already dry.
In this way, we can think of each new layer being more flexible than the one before it.
How to layer your painting using the fat over lean rule
To ensure your painting dries from the lowest layer up, slowly increase the oil content in your paint mixture in each consecutive layer of paint.
Keep your lower layers thinner (mixed with solvent) and only apply thicker, more oily paint in the uppermost layers.
There’s no right or wrong way to go about this, as long as you follow the general guideline. Different artists will have their own methods for layering paint and as you practice, you’ll find one you like.
The way in which you go about creating your process will be a personal one—it will depend on your desired outcomes and personal preferences.
I’ll walk you through some methods of layering fat over lean.
Different ways of layering fat over lean
Broadly you’ll paint with turpentine-thinned paint, then neat tube paint, then medium or oil enriched paint
It could be that you use only pigment and solvent for your first layer to block in the basic shapes in your painting. This is called painting imprimatura. Then in your second layer, you might thin your paint with turpentine. Then add a ratio of 2:1 turpentine to linseed oil mixture to your paint and then on top of that a 1:1 mixture and so forth.
You could also leave each layer to dry before applying the next. This means leaving at least one week before painting the next layer. This is an extra measure you can take to ensure the paint has had adequate time to start the curing process. It’s not that practical when working to deadlines, however.
You can take extra measures to speed up drying times for oily layers if you’re working to a tight deadline. One option is to add cobalt naphthenate to act as an accelerator. It’s best to only add a very small amount of cobalt to the medium you’re mixing it into—at most 50 drops of cobalt for 1 pint of medium.
Some artists don’t layer their paint at all, they paint alla prima (wet-on-wet) in one sitting.
There are some artists that opt to paint solvent-free and start with paint straight from the tube, then increase the oil content in the upper layers of their painting.
Think of the rule in relation to the brands of paint you use…
Not all paints have the same oil content; some manufacturers use little to no fillers in their paint, meaning that it has a high pigment and oil content possibly rendering them unsuitable for painting straight from the tube on the first layers of your painting in some cases.
Add too much binder and it could cause the paint film to wrinkle. Oil paint expands then contracts as it dries. This is especially true for linseed oil, not so much for other drying oils such as walnut or safflower.
What happens if you don’t follow the fat over lean rule?
If you paint over an oily layer with lean, inflexible paint, you are increasing the chances of there being structural problems with your painting. It will impact the appearance and permanency of your work.
As soon as it becomes touch dry, a lean layer over an oily layer will appear chalky. The glossy and well saturated layers below will appear to have a matted film stretched over the top.
Having reached the end of its curing process before the oily layers below, the lean paint will shift and crack as it is tightly bound to a moving surface. Parts of the paint will possibly come off the surface in places.
If you’re aware of the hierarchy of drying speeds and how solvent versus oil affects this, you won’t encounter any of these problems.
What about other mediums mixed with oil?
The rule differs slightly if you paint with alkyds. Alkyds like Liquin, are resins derived from acids, alcohols, or oils, so have a different chemical structure and therefore polymerise differently to oils.
When you add alkyd to oil, it dries quicker. It also makes the layer more flexible.
The flexible part is key here. We shouldn’t think of layering slower drying over quicker drying, but more flexible over less flexible.
So the rule with alkyds is to increase the amount of alkyd for each consecutive layer.
If you’re using alkyds in your painting, you should use them throughout, in every layer. Otherwise your painting can be subject to a phenomenon whereby the layers can just slide off one another and not adhere properly.
When using cold wax as a medium to thicken oil paint, you don’t need to worry too much about the fat over lean rule. You won’t be thinning your paint with solvent, only adding wax to create textured impasto effects.
Add the same proportion of wax to paint for each layer and you shouldn’t have any problems.
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