A former workshop attendee (not sure you can really call someone who spent three days under ones tutelage a student) just wrote to ask me about mediums. I’ll do my best to answer this one with what little I know backed up by the great Google. The fact is I don’t have a lot of real and accurate knowledge about mediums. But one general school of thought is the idea of painting lean to fat or thin to thick. Using more turp and less medium for the underlayers and more medium to no medium for the top layers. Reason being is that paint needs to dry throughout and if you seal off the top layers from oxygen the bottom layers may not get to dry and may eventually cause cracking…. just like with plate techtonics. What I generally do is to work the underlayers very thin, with turp and maybe a little medium, the middle layers are more medium and the top layers are no medium… painted more thickly.
I mostly use a fast drying medium called galkyd lite because I am impatient and want the underlayers to dry as quick as possible, galkyd is a resin based polymer and makes the paint dry really fast so I can go over it the next day or even that day with another layer or two. But this fits my painting style. I paint fast and loose and want underlayers to show through. Different mediums do different things and the one you choose needs to work with your painting style. If you want a lot of blendability in your paint for an extended period of time you use a medium that is designed to increase flow and slow down the drying time (see below). If you want to speed up drying time you pick something like galkyd or liquin.
Some artists use not turp only or no medium at all. Some just use odorless mineral spirits. Some use a mixture of 1 part linseed oil, one part turp and one part damar varnish which gives a nice flow to the paint, speeds up drying time and keeps the dark areas from dulling down, maintaining that still wet gloss. I wish I could go through all the variations and permutations of mediums but I only know just enough to be dangerous… I’d recommend some research on your part to find out what you need.
The following info on mediums is pulled almost verbatim from about.com with some editing from me.
“Linseed oil is made from the seeds of the flax plant. It adds gloss and transparency to paints and is available in several forms. It dries very thoroughly, making it ideal for underpainting and initial layers in a painting. Refined linseed oil is a popular, all-purpose, pale to light yellow oil which dries within three to five days. Cold-pressed linseed oil dries slightly faster than refined linseed oil and is considered to be the best quality linseed oil.
Stand oil is a thicker processed form of linseed oil, with a slower drying time (about a week to be dry to the touch, though it’ll remain tacky for some time). It’s ideal for glazing (when mixed with a diluent or solvent such as turpentine) and produces a smooth, enamel-like finish without any visible brushmarks.
Sun-thickened linseed oil is a created by exposing the oil to the sun to create a thick, syrupy, somewhat bleached oil, with similar brushing qualities to stand oil. Pour some oil (about an inch) into a wide dish, cover it with a propped-up lid (i.e. to minimise debris getting in, but so that the air can flow through). Stir every day or so to prevent a skin from forming on the top. How long it takes for the oil to thicken will depend on how hot the climate is where you live. Test the thickness of the oil when it’s cool, not when it’s still hot from the day’s sun. Pour it through a sieve or cloth to remove debris before you bottle the oil.
As linseed oil has a tendency to yellow as it dries, avoid using it in whites, pale colours, and light blues (except in underpaintings or lower layers in an oil painting when painting wet on dry). Stand oil and sun-thickened oil yellows very little.
Sun-bleached linseed oil is created by exposing the oil to the sun but with the container’s lid on, so no evaporation occurs. The result is an oil that has less tendency to yellow.
Poppyseed oil is a very pale oil, more transparent and less likely to yellow than linseed oil, so it is often used for whites, pale colours, and blues. It gives oil paint a consistency similar to soft butter. Poppyseed oil takes longer to dry than linseed oil, from five to seven days, making it ideal for working wet on wet. Because it dries slowly and less thoroughly, avoid using poppyseed oil in lower layers of a painting when working wet on dry and when applying paint thickly, as the paint will be liable to crack when it finally dries completely. Poppy seeds naturally contain about 50 per cent oil.
Safflower oil has the same characteristics as poppyseed oil, but dries a bit faster. It’s made from safflower seeds. Sunflower oil also has similar characteristics to poppyseed oil. It’s made from sunflower seeds.
Walnut oil is a pale yellow-brown oil (when newly made it’s a pale oil with a greenish tinge) that has a distinctive smell. As it’s a thin oil, it’s used to make oil paint more fluid. As it yellows less than linseed oil (but more than safflower oil) it’s good for pale colors. Walnut oil dries in four or five days. It’s an expensive oil and must be stored correctly otherwise it goes rancid (off).
Boiled oils are oils that have been heated and mixed with a dryer to create a faster-drying oil that gives a glossy finish. They tend to yellow and darken with age, so are best limited to lower layers in a painting and darker colours. If you’re not sure what effect an oil is going to have, rather take the time to do a test than ‘lose’ or ‘damage’ a whole painting.
One other note about liquin, it’s a great medium but I have noticed that it browns in the jar, though I’m not positive that translates into browning color, it’s something to think about. And never use Liquin as a topcoat or final varnish. It is a permanent, non-breathable, non-removable medium and as a straight layer, will yellow.