Classical Academic methods were used largely by the old masters during the Renaissance period and require a much more involved application of pigment. A complete underpainting ensures dimension and the results often times is that of a more unified even romantic feel. It's use today is much rarer in a world where very few are willing to take the time to produce a Renaissance piece of art. It is no wonder that the public is slowly becoming aware of its treasured value.
Plein Air is an approach to painting much favored by the impressionists, in which artists sketch outdoors to achieve a quick impression of light, air and color. The sketches are sometimes taken to the studio for reworking into more finished pieces or may be left untouched. They can also serve as references for larger paintings.
Alla Prima is a painting technique in which pigments are laid on in one application, with little or no drawing or underpainting. (Alla Prima is also known as “Wet-on-wet”.) It requires fast work because painting has to be done before the first layer has had time to dry.