You're in luck!
This post is a distillation of my seventeen years' search for the perfect medium for oil painting. All you have to do is read this and you will know everything I know, your paint will flow perfectly, and your paintings will look amazing. Trust me.
However, if this article is TLDR (too long, didn't read) just look at the photo captions and you'll get the gist.
To understand mediums (stuff to make paint flow the way you want) you have to understand solvents (stuff to thin paint and mediums). Oil Painters use solvents to clean paint off their brushes, both while they're painting – so that one color doesn't contaminate the next – and at the end of the day so that the brush can be effectively washed with soap and water. Oil painters also use solvents to thin mediums. We'll talk about that soon.
The traditional solvent was turpentine, which worked beautifully but had significant, toxic, smelly fumes. Now people use odorless mineral spirits (OMS) which is less smelly, and a bit less volatile, but still toxic. The least volatile solvent is Gamsol (shown above), which is what I use. I keep it in a brush washer to minimize fumes and I only open it when I need it. On the right is Turpenoid Natural, a non-toxic solvent, which can't be used to thin paint, so I use it only to clean dried paint on brushes and on clothes.
Medium and the Mysterious "Fat Over Lean"
I use medium for two reasons 1) to make the paint flow smoothly so I can make the brush marks I want and 2) to achieve "fat over lean" so that my paintings won't crack with age.
Fat over lean means using medium with more oil (fat) on the upper layers, and less oil and more solvent (lean) on lower layers of a painting. This makes the lower layers dry the fastest and prevents cracking because it prevents the creation of a skin on the upper layers before the lower layers are dry. So adding solvent to an oily medium makes it thinner and
appropriate for early layers of paint.
Of course, if you paint alla prima – that is if you complete the painting in one sitting – then you don't need to worry about fat over lean. However if you paint like I do, in several layers that dry between sessions, you need to use medium mixed with solvent in different proportions for different layers.
Tip: When the subject of oil paintings comes up at cocktail parties, if you nod knowingly and murmur, "Absolutely, fat over lean.", you will instantly be viewed as an art expert.
Mediums (what I don't use)
Mediums were traditionally combinations of solvent, oil, and sometimes varnish. Artists would make different mixtures depending on what layers they were painting. For a long time I would make three different mixtures of stand oil (very fat), linseed oil (fat), and Gamsol (lean). I would use my lean Medium 1 for the first layer, my fatter Medium 2 for middle layers, and my fattest Medium 3 for the last layers.
It worked just fine but it was a pain to measure out and mix up all the messy ingredients. Plus it was impossible to take outside for plein air painting, or to travel with.
Mediums (what I also don't use)
For a while I worked with this Walnut Alkyd Medium from M. Graham because it is non-toxic (can travel) and I didn't have to mix it (time saver). But it was very expensive and difficult to find.
Mediums (what I use!)
Then Gamblin came out with a new medium that solved all (well most) of the problems. It's called Solvent-Free Fluid and it's non-toxic, available everywhere and gives my paints the flow I want. I just pour it into a medium cup like the one shown, and if my paint feels a bit sticky or sludgy on my brush, I touch the tip of the brush into the medium, then touch it to a paper towel to blot, and then add it to my paint mixture. To achieve fat over lean I paint my first layers with a 1:1 mixture of medium to solvent (Gamsol), my second layer with 2:1 medium to solvent, and my top layer with straight medium.
For traveling and plein-air, they make a nifty tube gel version (on the right) that works well and stays on your palatte even when it's not level.
There You Have It
Aren't you glad you read to the end? I don't know about you, but for me, finding materials that work well and save me time or money is always a big deal. Painting is hard enough. :D
Please leave me a comment below and tell me about your favorite mediums. If you have any questions, don't hesitate to contact me here.