The principle of painting 'fat over lean' in oil painting is a rule one should follow to reduce the risk of an oil painting cracking. 'Fat over lean' has to do with the varying drying times of oil pigments (which can vary from a couple of days to a month) and ensuring that upper layers of paint don't dry faster than lower ones.
I did my usual. When I had the basic painting finished, I decided to glaze the background to help bring the pansy forward. I did this by thinning some pale green mix with some walnut oil. Now is this mixture fat or lean? I think it depends on whether walnut oil is fat enough. I'd appreciate any advise.
"White Face Pansy"
Oil, 6" x 6" on high quality deep canvas panel.