This article reviews some of the more popular oil painting mediums, their purpose, and some tips on how to use them. The purpose of adding these substances to your oil paints is to change the behavior of the paint during application and affecting results after the painting dries. Behavior refers to how the paint comes off the brush and glides on the surface, how it covers either the surface or succeeding layers, and just how it feels as you apply it.
Brands of paint act differently and mediums help you control the paint the way you want it to act as you use it. Some paint brands---and here I am only referring to the artist grade paints rather than the student grades---are stiffer right out of the tube. Student grades have less pigment and more fillers like extra oil and just do not perform well. If you use those paints that are stiffer out of the tube, but want more versatility in how they handle, or behave, you'll need a medium. Other artist grade paints are what I call fluffier and go on more smoothly right out of the tube. If you want brush strokes apparent in your final painting, a stiffer paint works better. Adding a refined linseed oil in tiny amounts until it feels right to you will encourage the paint to level out and show less strokes. Less linseed oil and more strokes will show. If you prefer an impasto technique (think Van Gogh), Gamblin Alkyd Gel thickens paint nicely. Always remember to never put a faster drying layer over a slow drying layer of paint. The top layer can dry too quickly and form a barrier causing the underlying layer to be sealed in and could ripple or crackle the surface down the road.
Glazing mediums allow you to apply thin layers of paint and build color and luminosity by having the viewer's eye mix the colors rather than mixing the paint on the palette or canvas. Using a medium like Liquin by Winsor & Newton speeds drying time while thinning the paint allowing layers to be built without waiting a few days for each layer to dry before you apply the next layer. There are also glazing mediums available like A traditional medium used for decades by many painters is refined linseed oil, a touch of solvent (typically mineral spirits), and a touch of stand oil, and a touch of Japan or Cobalt Drier These ingredients are mixed in a balance to achieve your desired results, like faster drying time, more gloss, etc. Stand oil is just a thicker linseed oil that can reduce brush strokes and increase gloss. Adding Damar varnish to your mix also adds gloss and can speed drying time. Damar varnish is made from tree resin and alkyd is a form of synthetic resin.
There are a number of mediums and I recommend you try several until you find what works best for your style of painting. Along with those mentioned above are safflower oil, poppy seed oil, and walnut oil.