Using Mediums
Mediums should be used sparingly in oil paint. There is a tendency to over saturate paint with oil, especially when using glazing techniques; demonstration videos on youtube are notorious for adding a whole bunch of oil to a tiny bit of paint. While there is no rule for how much you can mix your paint with, it's probably best to keep the amount of medium content under 1 part, per 2 parts of your paint from the tube, possibly even less. Adding too much oil makes the paint film somewhat unstable. Pigment particles actually act to strengthen a dried oil film. Its desirable to use just enough oil to make the paint behave as you want. If you find yourself adding what seems to be way too much oil to make your paint workable, it might be time to switch brands. Some artist prefer stiff paints and others a bit more fluid. If you like the feel of softer paints, look into brands such as M. Graham or Rembrandt.
There are a few ways to successfully add medium to paint. You can directly add it in drops like I do with a squeeze bottle or some artists prefer dipping their brush in a small cup of medium and dispensing to the paint. The last approach, most commonly found in glazing, is to apply a thin layer of medium called a couch directly to the dried surface of the canvas; paint straight from tube can then be painted into the medium on the canvas.
There are a few ways to successfully add medium to paint. You can directly add it in drops like I do with a squeeze bottle or some artists prefer dipping their brush in a small cup of medium and dispensing to the paint. The last approach, most commonly found in glazing, is to apply a thin layer of medium called a couch directly to the dried surface of the canvas; paint straight from tube can then be painted into the medium on the canvas.
Your job as an artist is to build a painting that will not crack by layering paint that is flexible over stiff or fat over lean. While it is probably ideal to follow the fat over lean by adding a bit more oil to each subsequent layer, there are no rule against fat over fat or lean over lean. Using a paint with a constant amount of oil in each paint layer should be safe practice as well. If your approach is one direct layer such as alla -prima, this rule doesn't apply.