I was recently told, that a fellow painter had the problem that paint was falling off his paintings after 3 years or so, so I asked why this might be and I was told that he apparently painted lean over fat. Since I have been learning to paint with oilpaint by myself, this was the first time I heard about the idea of painting "fat over lean". I did a bit of research, but still I have some questions.
If I have understood it correctly, basically painting "fat over lean" means to paint slow-drying layers over fast-drying layers, so that layers that are untop don't dry before layers that are underneath. This usually means to paint whith more and more oil.
Now I'm no expert in chemistry , but I would guess that dry layers lose flexibility and volume? Wich would mean that layers that are already dry and untop crack and start to fall off, as soon as layers underneath lose volume.
What if I paint over a layer that is already dry (let's say I let it dry for half a year or more, depending on thickness and pigment)? If I'm not mistaken, there should be no problem?
Also, what happens if I use a glaze? glazes usually dry fast. So I should not use a glazing-medium unless the underlying layer is completely dry? Supposedly Liquin adds flexibility to dry oilpaint wich on the other hand should mean that you could use it regarless. Alkyd-oilpaints are said to be very flexible after drying as well, nontheless I've read that you should not use them untop of normal oil-paints. So, what's the case?
thanks for the help
Oil paint expands and then contracts while it is curing. After is is dry it becomes progressively more and more brittle. This will cause a dry layer on top to crack if a layer below it is still drying.
However, fat over lean does not only refer to this. It also refers to how much binder (oil, alkyd etc) there is compared to the amount of pigment. For example, James McNeil Whistler used an extremely large amount of Turpentine in his paint. This caused a lack of binder and because of this the pigment is not being held onto his paintings and they are deteriorating. Fat over lean helps to prevent this because the extra binder in the fatter layer sinks into the thinner layer and helps to hold it on.
Alkyd and oil are two separate mediums. And although you can add alkyed into oil paint they dont really create a chemical bond to each other. So since no chemical bond is formed, putting pure alkyd over pure oil will not create a bond and the adhesion will be very weak (same issues occur if you put too much alkyd medium into oil paint though W&N denies it).