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Question About Fat Over Lean

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.


  • [Deleted User][Deleted User] admin
    edited November 2015
    jmac51 said:
    Doesn't the dry to the touch paint have less oil in it by definition than the new paint?
    No, I think this is what's confusing you. When oil paint hardens, it's because the vegetable oil polymerizes due to oxidation — the oil does not evaporate out. This is in contrast to mineral spirits, essential oil (like clove oil), or even water when using water-miscible paint, which dry by evaporation and are thus excluded from the final paint film.
  • SummerSummer -
    edited November 2015

    @jmac51 Let's say that you painted a layer and it is dry to the touch. Now you put another layer on top of that without adding more oil. Because it is the top layer, it dries fast. Soon you notice cracking. Why? Because the first layer is trapped and can't dry completely. If you had added a drop of oil to the second layer, both layers would still be drying at the same time. The dry to the touch paint SHOULD have less oil in it so it can dry faster.  The oil in the first layer doesn't evaporate, it just dries--which requires time and space to do so. At least this is my understanding of the process. Summer

  • Thanks for the replies everyone...David, I suspected that might be the case.

    I am greatly impressed, inspired and more than a little intimidated by my discovery of Draw, Mix, Paint. It has been 15 years since I have painted anything and as I said I worked in acrylic. Oils is such a different process. I am fascinated with detail and texture and the challenge of painting it. At least I was. But I have always wanted to be able to paint with a little more interpretive style, to say as much or more with a little less, and I find Mark's teaching style and method very compelling. But I worked slowly and lost gallery space more than once because I could not supply the number of paintings they wanted. 6 paintings is the most I ever finished in a 12 month stretch and I'm not sure how I could put together enough work when it takes at least 2 months for a painting to dry. Of course there can be no accumulation of work if one never starts. 
  • I was told by somebody at Windsor & Newton that "fat over lean" is really just a proxy rule for "more flexible over less flexible", which makes sense. Under layers shouldn't move more than layers on top of them or the layers on top will crack.

    Direct painting simplifies this though. :)
  • When you mix turpentine in paint--the paint thins and becomes LEAN.
    When you mix linseed  oil in paint--the paint thickens and becomes FAT.

    As you keep painting "Every new layer should have more linseed oil than the layer below it".
    FAT over LEAN.

    The underpainting or the Imprimatura (single color coating on the canvas) should have NO oil in it at all.

  • Folks you mean I didn't have to paint in all those cracks in my master studies?


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