In fresco painting it is necessary to have a well-thought-out design prepared before hand, as the medium does not permit experimentation or changes in drawing as is possible in most other ways of painting. If the plaster dries before it is completed, or if you have made a serious error, start over again with a new coat—a little bit of plastering experience will convince you that it is wiser to do the best you can with the piece before you—as plastering is really the only hard part of fresco painting.
Light, space, air and architecture are all determining factors in how the fresco shall be painted. The carrying power of the painting must be experienced in the actual setting, and it usually takes one patch, or less, to serve as the guinea pig in establishing the right key. This is one reason why a fresco painted directly on the wall, where it will always be seen, has a better chance for being right than does a mural painted on canvas in the artist's studio, and later glued to the wall.
On a large fresco toward the beginning of the work it is sometimes difficult to key the color. The first sections are so high and isolated that there is nothing to which to key the color, value and intensity. It usually takes one patch, or less, to serve as the guinea pig in establishing the right key. In addition as the work progresses the fresh new plaster is darker than other previous (and dry) patches next to it; it takes experience to trust that each daily section will match the previously completed ones.