Oil Painting - Guidelines
Painting over recently dried Underpainting

When methods used are more complicated than the simple direct oil-painting technique, such as a first underpainting which is allowed to set, or the use of thin glazes, or any multiple-layer technique, there is sometimes doubt as to the advisability of unrestricted free painting into or over surfaces that are wet, semidry, or completely dry. Investigators have suspected that some of these manipulations might lead to eventual cracking of the paint, because of interference with the normal drying action of the oil and the slowness of its conversion from the fluid stage to the tough hard film of dried paint. However, with well-made oil colors, correctly ground with the best linseed oil, it seems safe to paint freely in any of these ways, provided that the simple rules as outlined on page 131 are observed. There has been no objection on the part of chemists on the score of painting into or over wet or semidry or sticky paint, but the statement has been made--on the basis of [p. 109] some laboratory tests--that it is inadvisable to paint over recently or freshly dry coatings, and that once a paint film has become surface-dry it is better to wait until the layer is thoroughly dry before overpainting, usually several weeks.

This thorough drying is undoubtedly necessary with poppy oil colors, which should be used only for simple, direct, alla prima paintings, However, the best grades of linseed oil colors, which are less critical in this respect, will withstand this departure from theoretically correct treatment, and unless very heavy, enormously exaggerated impasto is freely used, artists find that they can paint over recently dried underpainting without bad results. [pp. 108-109]