The pigment was diluted with turpentine or linseed oil. When applied with a palette knife the pigment is not diluted. Generally, pointed brushes of sable are used for detail work and broad brushes of hog's hair or bristles for less detailed areas. Some painters use either a cool or warm underglaze that tends to show through over the years, especially when the surface paint is thin. A coat of varnish is applied to the finished painting.
Varnish tends to darken with age which is why many old paintings have required cleaning.
The Sistine Chapel ceiling fresco by Michelangelo is an example. Synthetic resins are now used. Originally, artists applied oil paints to wooden panels. Since wood has a tendency to warp or split canvas made either of linen or a linen-cotton blend was generally preferred. Acrylics are water-based paints made from acrylic resins.