The last layer, known as second painting, introduces medium and a thin more blended application of paint. As some of you know, this layer deals with subtlety and smooth transitions. It isn't needed everywhere, simply for more detailed or volumetric forms. Not all artists bother with a stage like this, as it is a preference determined by how you want your finished painting to appear. Sometimes I feel like skipping this stage as it causes people to walk up to my painting and tell me how much it looks like a photograph. I know they mean it as a compliment, but it's the equivalent of telling a fat person they look healthy or telling an ugly person they have beautiful eyes.
A couple painting tips.
1) Lead white #2 from Natural Pigments is so ropey, you can actually have a small string of paint hanging off of your brush which can be laid down as a long highlight. Keep in mind highlights are a variety of colors and shapes. I will also sometimes shape my highlights after they have been laid down with a clean brush loaded with oil.
2) To get thin crisp dark lines, like those found in wood cracks etc, I use mineral spirits to dilute the paint to an inky consistency, even though this breaks the fat over lean rule.
3) When starting with second painting, keep your colors the most vibrant in the beginning as they will grey down the more they mix.
I was too tired to write anything else witty, so here is a picture of a squirrel suplexing his friend.
p.s. Nobody can post a comment about how healthy I look with my pretty eyes.