The technique of painting in oil, the same way as the painters used until now, is to sketch with charcoal directly on canvas or in other ways being used a mixture of linseed oil with various solvents and pigments, thus yielding a thinner colour (which is ochre) that according to composition, with a drying time may be longer, depending on the desire of the painter.
The solvents that I was talking about, are the ones that we usually use in cleaning the brushes. As it is well known, rule number one in oil painting is to apply "thick over thin" that is always the last layer which normally (traditionally) should contain more oil than the previous layer, that can be applied by brush, however, techniques have evolved, after the need of the artist. Some artists work almost entirely with palette knife and occasionally helping themselves with the brush making it in more detail.Others are using a cloth or even their fingers. Oil paint has a long period of drying, thus allowing the painter to change the texture or shape the painting with a solvent, in most cases, turpentine. Oil paints have a drying period of 2 to 3 weeks in the film surface and from 6 months to a year in the second layer under the film, and the complete drying after the conservation of it, is from 60 to 90 years.
Oil paint is not drying up by evaporation but by oxidation.