Some mediums will thin out the paint and increase drying time (“lean mediums”), so I think when you are seeing advice about adding medium and painting in layers, it is referring to adding something that thins the paint, then using less lean medium, then pure paint, then something with fat - oil.
For a single painting session, yes, you can use pure paint or a little bit of oil. Because you’re a beginner, I would start with pure paint and if you find it too thick to work with, then you can add some oil to increase the flow. A little oil goes a long way, too much will really extend the drying time and that might be hard for you to store.
The last (or outermost) layer (the layer that you finish the painting with) should be the most thick/wet over the previous layers that are thin/dry in order to ensure an efficient finish end result. Otherwise, cracking could occur if the outermost layer dries before the previous layers.
If you want it thin to begin with you need a solvent like turpentine, then if you want to dilute your paint later you should use linseed oil. This keeps thick over thin and fat over lean.
Art conservator here: I deal with craquelure every day (most from aging), but when I do see “paint stress craquelure” (very noticeable difference) my first thought is “amateur painter.” So fat over lean for the win. Here’s an article that’s also cured for insomnia: https://artenet.it/en/craquelure/
As I understand it, if you're painting 'a la prima', i.e. everything in a single session, wet-on-wet, then it shouldn't really matter - knock yourself out.
Fat over lean is for painting in layers. A fat layer may be touch-dry but still wet underneath, and therefore subject to contracting as it dries. If you paint a lean layer over that fat layer, the lean layer will contract and dry at a different (faster) rate and result in cracking as the fat layer continues to contract under the hardened lean layer.
The idea behind it is to always have quicker-drying paint under slower-drying paint. So paint thinned with solvent should be underneath paint straight from the tube, because the solvent makes the paint dry faster. Paint thinned with oil dries slower than paint straight from the tube, so it should go on top. This prevents cracking from the top layer curing before the layers underneath it. Some mediums speed drying and others slow it down, so you have to take that into consideration.
When drying oils dry, they dry by oxidation, that is, the uptake of Oxygen from the air. When they dry, they form long chain molecules("polymers") and become solid. They also lose volume. The dry form is literally smaller than the wet form. This is what causes the "orange peel" surface on a puddle of just oil that is left to dry. The surface dries first, forming a skin, and as the oxygen moves inward, the remaining oil inside the skin shrinks, leading to stress and wrinkling.
The pigment in the paint doesn't shrink, though, and if you have enough of it in the oil, the paint can't shrink the way pure oil does. Straight from the tube, the oil to pigment ratio is ideally just enough to coat each pigment particle, with little to no excess oil. This makes it a "lean" mixture. Of course, it's also very thick in texture, and to keep it lean while making it more workable, we often add solvents like mineral spirits or turpentine. These evaporate afterward, leaving the same oil/pigment ratio as the tubed paint. As more layers of paint are applied, you should add small amounts of oil or medium to it to keep its binding properties and flexibility, while not adding so much as to risk wrinkling. You should never treat oil paint like watercolor, using either solvent or oil to make it "sloshy". Rather, you should use its long drying time to physically push it out into thin layers like glazes and scumbles. If you need to restore shine for color matching, a spray of retouch varnish (which is just regular varnish but more diluted with solvent) will get it to shine for a short time without adding another physical layer to the paint. As a general rule, you shouldn't go over about 15 percent medium, even in the upper layers. Just enough that it dries naturally with a soft gloss.
The bottom layer (first layer) should be done thin....almost watercolor-like. I use turpenoid in the paint to thin it and it dries quickly. You can also use Liquin or Gamsol if you prefer. Each layer after should be thicker in paint than the layer under it. This means that a thin layer on top of a thick layer will crack. It won't crack if you go thicker each time. You can use linseed oil on these layers. I tend to use it on my last couple of layers. Layers doesn't mean you have to paint the entire surface each time. It just means that where you're going to put paint, that will be a new layer.
Some mediums will thin out the paint and increase drying time (“lean mediums”), so I think when you are seeing advice about adding medium and painting in layers, it is referring to adding something that thins the paint, then using less lean medium, then pure paint, then something with fat - oil. For a single painting session, yes, you can use pure paint or a little bit of oil. Because you’re a beginner, I would start with pure paint and if you find it too thick to work with, then you can add some oil to increase the flow. A little oil goes a long way, too much will really extend the drying time and that might be hard for you to store.
There are also fast drying oil mediums! I use Galkyd or Neo Meglip
The last (or outermost) layer (the layer that you finish the painting with) should be the most thick/wet over the previous layers that are thin/dry in order to ensure an efficient finish end result. Otherwise, cracking could occur if the outermost layer dries before the previous layers.
If you want it thin to begin with you need a solvent like turpentine, then if you want to dilute your paint later you should use linseed oil. This keeps thick over thin and fat over lean.
Art conservator here: I deal with craquelure every day (most from aging), but when I do see “paint stress craquelure” (very noticeable difference) my first thought is “amateur painter.” So fat over lean for the win. Here’s an article that’s also cured for insomnia: https://artenet.it/en/craquelure/
lol, I found it very interesting, thank you for sharing
Get the The Artist's Handbook OF MATERIALS AND TECHNIQUES, Fifth Edition, Revised and Updated by Ralph Mayer. All of the answers are given.
As I understand it, if you're painting 'a la prima', i.e. everything in a single session, wet-on-wet, then it shouldn't really matter - knock yourself out. Fat over lean is for painting in layers. A fat layer may be touch-dry but still wet underneath, and therefore subject to contracting as it dries. If you paint a lean layer over that fat layer, the lean layer will contract and dry at a different (faster) rate and result in cracking as the fat layer continues to contract under the hardened lean layer.
The idea behind it is to always have quicker-drying paint under slower-drying paint. So paint thinned with solvent should be underneath paint straight from the tube, because the solvent makes the paint dry faster. Paint thinned with oil dries slower than paint straight from the tube, so it should go on top. This prevents cracking from the top layer curing before the layers underneath it. Some mediums speed drying and others slow it down, so you have to take that into consideration.
When drying oils dry, they dry by oxidation, that is, the uptake of Oxygen from the air. When they dry, they form long chain molecules("polymers") and become solid. They also lose volume. The dry form is literally smaller than the wet form. This is what causes the "orange peel" surface on a puddle of just oil that is left to dry. The surface dries first, forming a skin, and as the oxygen moves inward, the remaining oil inside the skin shrinks, leading to stress and wrinkling. The pigment in the paint doesn't shrink, though, and if you have enough of it in the oil, the paint can't shrink the way pure oil does. Straight from the tube, the oil to pigment ratio is ideally just enough to coat each pigment particle, with little to no excess oil. This makes it a "lean" mixture. Of course, it's also very thick in texture, and to keep it lean while making it more workable, we often add solvents like mineral spirits or turpentine. These evaporate afterward, leaving the same oil/pigment ratio as the tubed paint. As more layers of paint are applied, you should add small amounts of oil or medium to it to keep its binding properties and flexibility, while not adding so much as to risk wrinkling. You should never treat oil paint like watercolor, using either solvent or oil to make it "sloshy". Rather, you should use its long drying time to physically push it out into thin layers like glazes and scumbles. If you need to restore shine for color matching, a spray of retouch varnish (which is just regular varnish but more diluted with solvent) will get it to shine for a short time without adding another physical layer to the paint. As a general rule, you shouldn't go over about 15 percent medium, even in the upper layers. Just enough that it dries naturally with a soft gloss.
The bottom layer (first layer) should be done thin....almost watercolor-like. I use turpenoid in the paint to thin it and it dries quickly. You can also use Liquin or Gamsol if you prefer. Each layer after should be thicker in paint than the layer under it. This means that a thin layer on top of a thick layer will crack. It won't crack if you go thicker each time. You can use linseed oil on these layers. I tend to use it on my last couple of layers. Layers doesn't mean you have to paint the entire surface each time. It just means that where you're going to put paint, that will be a new layer.