"I really want to start painting in oil. It has been a dream of mine for as long as I remember. I don't intend to paint professionally, just for my own satisfaction. I finally have the chance to do it, but my enthusiasm has hit a wall and I'm pretty confused on the choice, use and application of mediums..." — Masha
Oil Painting Method
There are as many ways to paint as there are artists, but here's a summary of my oil painting method. To begin with, there are two simple rules you should follow. Firstly, you need a surface to paint on that has been prepared specifically for oil paints. You can buy many brands of canvases, and if you are really willing to spend some money, use linen canvases. Most come already prepared (check the label, or ask).
Secondly, when you apply the paint you must follow the rule of fat over lean, which means the paint you put down first that comes first is 'leaner' (has less oil) than the subsequent coats (which in turn will have more and more oil). Let me explain how to achieve this.
The first coat of paint you should dilute the paint with your chosen solvent. I recommend using an odorless solvent. You will have to have a very good ventilation anyway -- even though you are not smelling it, it still evaporates. Dilute the paint until it has the consistency of a watercolor (that means like melted butter) and fill in the areas with this paint using a stiff brush.
The size of the brush to use varies with the size of the area to be painted. I recommend using a lot of brushes when painting. If possible, one brush for each mixture of paint.
The next coat of paint, which will be applied after the first is dry, will have less solvent added. (Don't add any oil yet.) Your paint will have a creamy consistency, slightly more diluted than the tube consistency. On this stage you will cover the earlier coat with more consistent paint and begin what's called modeling. That is, you will soften transitions between areas, define more or less hard edges, darken the shadows and lighten the lights, but nothing definite yet. Leave some room to modify later. Don't paint in the darkest darks nor the lightest lights yet. Wait until it dries.
The next coat will take the longest. You can use the paint without any medium, at the consistency it comes out of the tube (though some artists like to soften the paint a little). Unlike the other first two coats, in this coat, if everything is right, you won't have to cover all the canvas and will be able to work on sections. Work carefully and take your time. Depending on the painting and your working pace it can take from few hours to several days. You can define more the lights and shadows. When you're done, you will be close to finishing the paint. Wait until it dries.
The next coat (or coats) are the finishing ones. You will add a small amount of linseed oil to the paint to follow our golden rule: 'fat over lean'. (Stand oil is another option; it's an oil that is modified and yellows less than standard linseed oil. It also cracks less.) If you want to add a siccative to speed up the drying time of the paint, I suggest you to use Liquin, a synthetic resin that makes the paint dry faster and is fairly safe. I've been using the following mixture for years without any trouble: 1 part Liquin, and 1 part composed of 1/2 part stand oil and 1/2 part odorless solvent. Shake it until it mixes and it is ready.
You will see the paint is slightly transparent due to the medium, which is desirable because on these stages you will only modify what's already on canvas, defining the lights and darks (finally!), and modeling a little more. You can use as many coats as you want, but remember, the less, the better, because you will have less probability of the paint changing over time. The less you mess with the paint's original consistency adding oils, the better.
Remember: when you are beginning, anything goes. Feel free to experiment. Try different combinations of paint and medium until you find one that suits you best. The same goes for the brushes. And practice as much as you can!