Art is filled with aphorisms. Fat over lean, for instance, is a famous adage. The meaning for this old saw that is the most straightforward is to paint thickly-applied paint over thinly-applied paint.
Why this distinction? Well, if you paint in layers, that is, if you paint over previously painted areas, you ignore this practice at your peril. Thick paint dries slower than thin paint. Thin, fast-drying paint applied over thick, slow-drying paint can crack and spoil the work. This problem can be exacerbated if you use driers inconsistently. If you use driers–an altogether sound practice–use them consistently in every layer.
If you are an ala prima painter you might believe that the fat over lean adage doesn’t apply to you. But there is an important way that this applies to you too. Another adage, make the lights thick but make the darks transparent, amplifies the other saying by emphasizing an aesthetic point. Alternating thin and thick areas
But it’s more than just variety. The eye reads areas painted over other areas as closer to it. Those closer areas need to be thick enough to hide what’s beneath. Paintings that fail this basic mode lack body and appear insipid and timid.
A lot of artists instinctively understand this but overcompensate by painting everything uniformly thick, which robs the painting of charm and, counter-intuitively, force. Thick areas as shown to best effect when contrasted to thin ones. Painting everything uniformly thick robs a painting of several keys. It’s like restricting yourself to just a third of the keys on the piano keyboard.