[Michael Fournier]

Karin, Great info. I wanted to add a few additions that also might help.

The first one is not all mediums dry at the same rate nor do all paint colors. So you must be careful when using different additives to your paint and when you have heavy applications of slower drying paints that you do not put a faster drying layer over a slower drying one.

The rule of Fat over Lean assumes that the Fat layer of paint contains the same or similar type of oil-based medium as the lower layer.

All oil paint contains oil so even directly from the tube it can be fat depending on the layer you intend to apply over it.

If you use linseed, almond or similar oil-based mediums then as you add more medium to your paint it is Fat, meaning that it contains more oil.

But, here is the gotcha:

Alkyd resin mediums like Liquin which are quick drying will dry much faster than oil paint direct from the tube or with added oil-based mediums. If you were to follow the rule of Fat over Lean, assuming that the paint with more medium (Liquin in this case) was Fat then you would be in trouble if the layer you are painting over was thick oil-rich paint direct from the tube, even with no oil medium added.

Some mediums can contain other drying agents like Japan Drier which also hasten the drying of oil paint. (Caution: Japan Drier can cause paint to become brittle and should be used very sparingly if at all.) These mixes also should not be used over an oil-rich layer.

Even if you waited until the oil rich layer was dry to the touch it still is likely to shrink more than a faster drying layer over time, causing the top layer to crack or worse even flake off.

You could put a layer of paint that is 50% Liquin down as an initial layer of paint and it would dry very fast. You could paint over it with paint direct from the tube with no medium added and it would still be following the fat over lean rule. Why? Fat means "more oil" and paint from the tube has more oil than a 50/50 mix of paint and Liquin and dries slower.

And one more note: paint thinned too much using just solvent, either turps or an odorless substitute (most are mineral spirits) is a very weak layer of paint and you should be careful not to thin top layers too much.

On bottom layers it is ok (many artists use an oil wash to tone their canvas) since the loose pigments will combine with the subsequent layers of paint and bind with the oil in those layers.

When an oil wash is to be left visible or as the final layer it is best to thin the paint with some kind of medium that will bind the pigment. I use Liquin for this because it dries fast and because the alkyd resins in it bind the paint to the surface.

I have done oil wash paintings using all transparent layers thinned with Liquin with the top layers having less Liquin and more oil. None have cracked or have had any separation between layers. (The oldest have only been around for 10 years so I cannot say what might happen in 50-100 years from now.)

The best advice beyond what Karin has given is: Think of "Fat" as "slow drying" and "Lean" as "fast drying". Then if you follow "fat over lean" you should be ok.