‘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.
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Fat over lean: What is lean paint?
What is ‘lean paint’?
Lean paint is paint that has been mixed with a paint thinner such as turpentine or oil of spike lavender.
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….
Fat over lean: What is fat paint?
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 the fat over lean 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. Or you could choose to paint straight from the tube in each layer.
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.
Fat over lean mediums
As you can see, the oil painting process is flexible and there are many methods of painting which adhere to this rule. Just make sure you don’t paint with solvent over an oily layer, and you should be fine!
The Fat and Lean mediums by Chelsea Classical Studio are made especially for artists wanting to use this technique. They are made from oil of spike lavender, damar resin and linseed oil. The unique combination prevents the paint film from wrinkling and gives a wonderful satin finish. These are fine art mediums and feel luxurious to use. They’re studio safe and don’t have carcinogenic fumes associated with solvents like turpentine. To use the Chelsea Classical Studio mediums, start your painting by adding a few drops of the Lean Medium, then increase the amount of Fat Medium with each layer. I would especially recommend these mediums for artists who like using the glazing technique and use softer brushes to achieve realistic effects.
Think of the rule in relation to the brands of paint you use…
Not all paints have the same oil content; some manufacturers like Michael Harding and Schmincke use little to no fillers in their paint. This means that they have a high pigment and oil content. If you use these brands to paint straight from the tube in the first layers of your painting, they will dry slowly. If you have a mixture of student grade and professional grade brands, of course it’s fine to mix them together. It does make sense to use the professional paints in the final layers of the painting, as they are more pigmented and oil rich.
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.
Fat over lean: Pin it!
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8 thoughts on “The Fat Over Lean Rule in Oil Painting: Explained”
Hi loved the article realy informative.
My question is in theory you could start painting fat and use the same mixture through each layer and the same if you wanted to paint lean say … as all the layers would be drying at the same rate ? For exsample i use 1 : 1 ratio of solvent to linseed oil but use it for layer 1,2,3,4 .. ext
Am I correct in thinking this I would love to hear ur opinion ?
Hi Thomas, thank you! I’m glad you found the article helpful.
Yes I think it would work perfectly to use the same proportion of medium mixed with your paint in each layer.
Brilliant explanation thank you! I have some Resin Oil medium which is a combination of Beeswax, Dammar Resin, linseed stand oil and double rectified turpentine. I’m using this should I add it throughout to each layer or use solvents first and add this to the final layers in increasing percentages? Can it be used over fatty layers where I have already added linseed oil and what is it’s flexibility like?
Hi Jo, the resin wax medium will create a flexible film and can be used over paint layers that have been thinned with solvent if you wish.
Brilliant explanation thank you. Does the linseed oil work as same as liquin does in terms of the glazing method on oil paint?
Linseed oil is runnier than regular Liquin and it will dry slower, but has similar properties in terms of increasing transparency of the paint film to make glazes.
Great article! I have one specific question I’m not able to find any knowledge on.
I’m trying to paint solvent free and just working with stand oils to dilute (don’t have a great ventilated space). I’m basically just confused about the fat over lean principle when painting the underpainting with a thinned out paint. If I paint the underpainting with paint thinned with walnut oil and then build up the rest of the painting in one go with leaner paints (building up to straight out of the tube) does this increase the risk of the painting cracking as it dries? What if I paint another fatter layer on top after it’s touch dry, is this problematic long term?
Hi Jonathon, if you want to make your underpainting fluid, solvent-free, lean and fast drying, one option is to use water mixable oil paints thinned with water. You can paint on top of water mixable oils with regular oil tube paints, and oil paint mixed with other oil mediums. This is a good workaround, as it means you can layer thinned paint before fatter layers. When you add water to water mixable oil it reduces the oil content, essentially acting as a thinner to create a lean layer. Walnut oil will make paint runny, but also more oil rich and ‘fat’ so I wouldn’t advise using that for an underpainting.