I know this is basic stuff but I come from an acrylic background and am confused about something concerning the need to paint fat over lean. As a general concept I understand it but if you begin a painting and have to leave it for a time and that layer of paint is now dry to the touch, why do you need to add oil to the new paint? Doesn't the dry to the touch paint have less oil in it by definition than the new paint? Especially if using Mark's Geneva paint which already has medium in it. It seems that if one layer is dry to the touch and and new paint goes over it, you are already painting fat over lean. What am I not understanding? Thanks for any comments.