Properties of Oil Paints
“Fat over Lean”
Oil painting can be as simple or as complicated as you want to make
it. There are all kinds of surface preparations, mediums, solvents,
varnishes and other finishes you can experiment with.
Personally, I prefer to keep things as simple as possible and devote my time and effort to the painting process itself. Still, it’s
nice to try something different now and again.

No matter what experiments you want to try however, there are a few basic principals you need to be aware of. Proper
preparation of your support is one thing. We deal with that elsewhere. The other is the commonly-heard principal of “fat
over lean”. What this means is that when you are applying one layer of paint on top of another, the layer on top should
have a higher oil content than the layer underneath. The reason for this is you want to avoid situations where a layer of
paint dries faster than the layer underneath. As the layer underneath continues to contract you will get cracks in the top
layer.

To understand how this works in practice, you have to understand the difference between oils (such as linseed oil) and
solvents (turpentine and its various substitutes). Both can be used to thin or “cut” the paint. Paint squeezed straight from
the tube already has linseed oil in it. If you add more oil, you are increasing the “fat” content. If you add turpentine
however, you are spreading the oil in the paint over a larger surface area.  You are therefore reducing the oil content
(making it “lean”).

Thus, the normal process is to use turpentine (or equivalent) generously to thin the paint during the underpainting process.
In the main body of the painting you can use a mixture of turpentine and linseed oil (or a painting medium such as Liquin) or
work with paint straight from the tube. If you are using a glazing technique, you would once again want to thin the paint,
but this time you would use just linseed oil or liquin.