Bill Martin's Guide to Oil Painting
All paint is a mixture of a dry pigment and a liquid. For oil paint the dry pigment is mixed with linseed oil. Linseed oil is a drying oil that dries by oxidation. That is, it takes oxygen from the air and creates a crystalline solid that encases the pigment in a permanent form. When it's dry it can't be removed.
Oil paint is thick. It comes in tubes. The paints are squeezed out onto a palette where they are mixed together with a palette knife to make the various colors. It's then applied to a vertical canvas with stiff brushes.
Oil paints are slow drying. Usually taking three days before you can put on another coat. The drying time is an advantage and disadvantage. The great advantage is that you have time to refine and adjust what you paint before it dries. This is particularly useful when making gradual transitions from one color to another. Also if you paint something you don't like, it can be removed while wet using a rag, palette knife or rubber squeegee and replaced.
The disadvantages are that when putting two wet colors next to each other they can cross-contaminate if not applied accurately. The palette, the brushes and the wet painting must be handled carefully to keep wet paint off, fingers, food, fabric and furniture.
The paint may be manipulated for up to 12 hours after which you must wait three days for it to dry before going back to make any changes. When oil paint is dry, new colors can be applied over old. Many layers of paint may be applied. If the paint is to be used thickly each layer should be as thick or thicker than the previous layer to avoid cracking.
After a painting is thoroughly dry (three to six months), varnish is applied to protect the painting.