(1)I confused about fat over lean, some colors' drying time faster and some slow. If I make an painting in allaprima/wet on wet technic and paint sky with mixing of two or three colors of different drying time for example cobalt blue, phthalo blue and zink white, will it crack ?.
(2)Painting by layers, if first layer of painting made by colors of different drying time when i paint next layer it should be on touch drying time or more?.
(3) glazing. someone suggest that a thin layer of paint should be use for glazing but fat over lean says avoid thin paint on thick layer.
Kamangar, great questions! Opaque colors dry a little quicker than translucent colors because they contain less oil. But don't worry if you mix them together, the resultant color won't crack as it dries as long as they are thoroughly blended. For questions 2 and 3, my advice is to let the layers dry completely, then apply a light wash of stand oil before you paint another layer. The thing to keep in mind is apply more oil with each subsequent layer. It's when you do the opposite is when you run into trouble! Happy painting!