The most common question I get about oil painting is in regard to “fat over lean”. I went years and years without understanding how to best utilize this technique and I’m still learning new aspects. Even if somebody gets the general idea, there’s still much that isn’t understood when you’re sitting in front of the canvas.
The main idea is this: oil painting requires oil and oil is a tricky substance that “dries” in a unique way, requiring an approach that will get the best results in terms of aesthetic and preservation.
A basic understanding of oil paint will help. Oil paint is essentially pigment (from rocks, plants, minerals, etc) suspended in oil (as the binding agent – the thing that holds it all together). Adding oil to the paint will alter the ratio of pigment particles to oil mass, with the former reducing in relation to the latter (helping things move more smoothly and aiding in transparency). Solvent, on the other hand, will do the opposite, eating away at the oil and leaving a ratio in favor of pigment. To really understand how this works, I would encourage you to try it out on some cheap canvas and see the difference between adding more oil or solvent.
To get the best results from painting fat over lean, thin the first layers of your painting with a solvent like Turpenoid or Odorless Mineral Spirits (you can use the thinner in combination with a little oil medium to ensure that it doesn’t get too thin). As you lay on more layers, you can start to incorporate more oil, whether you’re painting in distinct layers or using a more wet-into-wet approach. What you’re hopefully achieving will lead to thicker application of paint as you progress (note that I’m not saying thicker application of pigment).
That’s the gist. Try it out yourself and see your results.
As an added bit of useful information, take note of the differing transparency levels of your paints. Some pigments are more prone to being transparent and some are more opaque. This means that some paints will react to the addition of oil differently than others. For instance, Titanium White is very opaque. Adding oil will increase its flow, but will not generally make it more transparent. Payne’s Gray is very transparent. Oil will help it to “wash” over earlier layers in a way that can add luminosity and depth of tone. As you learn these elements, you’ll begin to utilize your colors much more effectively and broaden your abilities.