I recently decided to leave my arty comfort zone as a watercolorist and experiment with an entirely new-to-me medium – oil painting. I realized as I experimented that there is a lot more to oil painting than simply applying paint to canvas.
One of the first things that I learned was how to thin the paint. Here is what I learned.
To thin or change the consistency of your oils, you can use a solvent as an additive to give your paint a lower viscosity, a slightly quicker drying time, and a matte appearance. If you add natural oils, your paint will have a higher viscosity, glossier results, and a slower drying time.
The additive you choose to use depends on your painting technique and the effect you want to achieve in your painting. Let’s explore the additives you can use to change the consistency of the paint.
Additives to Make Oil Paint Thinner
Mineral spirits or turpentine are the most basic solvents to thin down your oil paint. The amount you add will affect the consistency of the paint. They are also usually much cheaper than other mediums and oils.
More mineral spirits mean a watery consistency, whereas a small amount will make the paint buttery. So if you want to cover your canvas with paint quickly, the thinner consistency is the way to go.
The Fat Over Lean Rule
When you add solvent to the paint, the resulting mixture is “lean,” and when you add a natural oil to the paint, it is called “fat.” One of the golden rules in oil painting is “fat over lean.”
Essentially, this rule means that you will always start your painting with layers of lean paint and finish it with “fat” layers. Therefore, you need to ensure that there is at least the same or less solvent and more oil in every new layer of paint that you add to your painting.
Don’t thin your paint with a solvent if one of your previous layers was done with thick, oily paint.
Following the fat over lean rule will prevent the paint from cracking or “sinking in” after the painting has dried. “Sinking in” is the term for the dull spots that may appear on an oil painting.
Turpentine vs. Mineral Spirits
Mineral spirits are derived from petroleum distillates, whereas turpentine comes from pine tree resin.
They both have the same effect on oil paint, but the fumes from turpentine might be harder to tolerate, and the cost of turpentine is often higher than mineral spirits.
The Downside of Using Solvents
One of the downsides to solvents are the fumes. They can be harmful to breathe in over an extended period of time, and some people can’t use them at all.
To counter this, consider using artist-grade solvents, for example, Gamsol or Turpenoid. These have no additives and less smell.
If you are going to use cheaper mineral spirits, make sure that you have plenty of ventilation. Better still, work outside.
How to Use Solvents to Thin Your Oil Paint
The following technique to thinning your oil paint will help prevent excess fumes and evaporation:
- Pour a little solvent into a small jar with a tight-fitting lid. Solvent quickly becomes dirty, so using a small amount means you can often replace it with clean solvent.
- Dip your paintbrush into the solvent. To thin your paint slightly, just dip the tip of the brush in the solvent. For a more watery consistency, dip the whole brush in the solvent.
- Mix the solvent and paint on your palette.
- To thin a large amount of paint, sprinkle the solvent onto the paint on your palette.
- Repeat the processes above until you have reached the desired consistency.
Using Oils to Change the Consistency of the Paint
We spoke earlier about the “fat over lean” rule. When we mix the paint with another oil, it becomes “fat.” So while changing the consistency, it is technically not thinning the paint.
If you prefer working with thinner paint, you can thin your first few layers with the solvent, and then the subsequent layers can be mixed with oil. Oil is also suitable for a glazing technique.
Adding oil to the paint results in higher viscosity and glossier finish, and keeping with the rule, make sure you only use the “fat” paint in the last layers of your painting.
Types of Oil
Linseed oil is a very commonly used oil. It is stable with an average drying time. However, it can yellow over time so for lighter, cooler colors, consider a different oil.
Walnut oil is very resistant to yellowing and cracking, and it dries slowly. It also adds some intensity to your colors.
For oil that dries quite slowly and evenly, safflower oil is a good option. However, it does feel sticky while drying as it doesn’t dry from the outside in, like linseed oil. It also doesn’t yellow like linseed oil.
For the slowest drying time, poppy oil is a great option. But don’t use it in your initial layers as there are concerns about its stability over long periods.
Stand oil is basically linseed oil that has been heated, resulting in a thicker, higher viscosity oil. As a result, it dries faster than regular linseed oil and doesn’t result in as much yellowing.
How to Use Oils to Change the Consistency of Your Oil Paint
Follow the same method as with the solvents, although the lidded jar is not as much of a concern as there aren’t the same fumes to contend with.
The Last Note – Mediums and Gels
We have spoken about solvents and natural oils. Aside from these, there are various manufactured mediums and gels which you can use to change the viscosity of your paint.
There are many different ways to change the viscosity of your oil paint. We hope this guide has given you a starting point to learning how to thin your oils. Just don’t forget the golden rule: “fat over lean.”