The Principal of Fat Over Lean

Some oil paints, especially student grade, contain fillers, such as chalk and additives, that thicken the paint and sometimes make it hard to apply. The first inclination is to thin the paint with odorless mineral spirits such as Gamsol, but be careful! In the early stages of a painting, thinning the paint with mineral spirits is okay as long as the ratio of oil to mineral spirits is no more than 50/50. As you build more layers of paint, however, you want to lessen the amount of mineral spirits and increase the amount of oil (if the paint needs thinning). 

Some artist grade paints have plenty of oil, so the addition of oil or mineral spirits is not necessary. Thinning paint with mineral spirits tends to weaken the paint and makes it susceptible to cracking. It will also tend to ‘sink’ the colors, causing the gloss in the paint to flatten and become dull. If you apply paint that is thinned with mineral spirits over a fat layer of paint, the upper layer will crack because it will dry faster than the layer under it. Use a refined or cold pressed linseed oil to extend the viscosity of the paint as you build layers, but use only enough to make the paint easy to apply. Both are thin oils, versus a stand oil, which is thick and dries slowly.