thick on thin rule
Oil Painting for Beginners - Rules
The 'Thick on Thin' Paint Application Rule
After mixing your palette, application to the canvas is the next step. I suggest you do most of your colour mixing on the palette rather than the canvas, as a muddy effect will result with too much mixing on the canvas. Unless of course this is the effect you are after.
Generally oils are meant to be worked wet on wet in layers. This may not be practical as your work may tack off during the process. Acrylics are better designed for wet on dry painting. If you are working in layers of oils then you must place the top layers of paint on heavier than the under layers.
Oils do not evaporate when they cure, they oxidize. That is the oils mix with oxygen to cure. Acrylic paint dries by evaporation. If a thin layer of oil is placed over a heavier layer then the picture may be prone to crack.
An oil painting may take anything up to 12 months to cure properly. The term used for heavy oil painting is 'IMPASTO'