Thinning Oil Paint: A Guide

Thinning oil paint: how to dilute and thin artists’ oil paint to make it runny in consistency and clean it from materials.

Oil paint is different from other mediums in the sense that it is not water soluble. Instead, painters may use a solvent to thin colours. 

Using solvent to thin oil paint is perhaps the most common way to do it, but it does come with some drawbacks. Luckily, it’s possible to thin oil paint without solvent too. I outline some of the options available to artists who want to dilute paint, clean brushes and change the viscosity.

Thinning oil paint: which solvents can you use?

There are four main types of solvent you can use with oil paint. Some of them are more compatible with paint than others.

Despite there being different types, solvents all have similar working properties. They speed up drying time and dilute paint, reducing viscosity. 

If too much solvent has been mixed into paint, it can make dry paint appear chalky. Using solvent in a paint mix reduces the oil content, which creates a weaker paint film. So take care when using it, not to add in too much.

Turpentine

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Pros: highly compatible with paint, strong solvent, cheap, fast drying

Cons: strong heady fumes, can be bad for health

One of the most compatible solvents to use with oil paint is turpentine. It’s also one of the strongest solvents. It thins paint effectively, leaving a layer of pigment dispersed evenly throughout. The solvent is strong enough to dilute varnishes such as damar resin. 

When buying turpentine, ensure it’s the artist grade distilled variety, as decorator’s solvent can have unwanted additives.

Historically, turpentine is the most widely used solvent. This is due to its availability and therefore cheap price point. The solvent has been used by artists for hundreds of years, so the effects (both on a painting and on people’s health) is well documented.

Turpentine is actually a resin, distilled from pine trees. The smell of turpentine is strong, but fragrant, with notes of pine. However the fumes that turpentine emits are its main drawback. They can cause headaches, but can also cause longer term health effects if not used correctly.

The toxicity of the thinner is pretty high, so I would only use this solvent if you have a large space to paint in with good ventilation. To ventilate a space properly, you need two windows you can keep open to allow air to circulate.

The fumes (and aromatic hydrocarbons it contains) is what puts many artists off of using turpentine, now that there are so many alternatives available.

There are ways to prevent negative health effects when thinning oil paint with solvent. On top of ventilating your space properly, you could buy a brush washer to contain the turpentine so that the fumes aren’t leaking out into your room whilst painting. Some people are more sensitive to turpentine’s fumes than others, I tried using turpentine as a medium for a while but very quickly started getting headaches. So I stopped using it and switched to oil of spike lavender.

Odourless Mineral Spirits

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Pros: studio safe variety, cheap, fast drying, great for cleaning

Cons: not as effective at thinning paint as other solvents

Artists’ quality odourless mineral spirits (OMS), which is also called white spirit in the UK, is a petroleum distillate.

Gamsol, which is Gamblin’s formulation of OMS has been refined to include less of the aromatic hydrocarbons that can be harmful. For this reason, it’s marketed as being ‘studio safe’. Whilst it is deemed more studio safe and has lower toxicity than turpentine, spaces must be ventilated properly as it does still contain small amounts of those unwanted chemicals. As with turpentine, though, getting a good brush washer will seal the solvent, preventing the fumes from contaminating your studio.

OMS isn’t as compatible with paint as oil of spike lavender or turpentine, however it still works effectively. It’s a great option for cleaning brushes.

You can mix OMS with any type of drying oil, such as linseed, poppy, walnut or safflower to make your own medium. By mixing in extra oil medium with OMS, you can ensure the paint film will remain strong and flexible.

Oil of Spike Lavender

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Pros: most effective paint thinner, non-toxic, most compatible with paint, pleasant aroma

Cons: more expensive to buy

There are many advantages to using oil of spike lavender as a solvent. It’s the only non-toxic solvent and it’s non-carcinogenic unlike other thinners.

Oil of spike lavender is a plant based, organic solvent that has a pleasant aroma. 

The thinner dries slowly compared to other solvents and the finish of dry paint mixed with this medium is satin rather than matte.

It’s more expensive, but you only need to incorporate a tiny amount of the liquid into your paint mix, due to its effectiveness at thinning. It will thin resin, as well as oil, so you can use it with damar resin to create varnish.

This is the solvent I choose to use. Oil of spike lavender can be mixed into the first layer of my paint to speed up drying time. I don’t use it to clean brushes, I opt to use brush soap instead.

Chelsea Classical Studios create some wonderful premixed oil mediums which include oil of spike lavender. They are high quality and feel luxurious to use.

Citrus Solvent

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Pros: Less toxic than turpentine, more environmentally friendly

Cons: Less information available about its effects and properties, strong smell

This is another option for artists, which is relatively new solvent formulation. It’s safer to use than turpentine, and has a citrus smell.

The ingredients are citrus oil and hydrocarbons, it mostly contains D-limonene.

Zest-it citrus solvent is also described as environmentally friendly as it’s biodegradable. It can cause skin irritation, so wear gloves when handling.

Is solvent safe to use?

When breathed in, solvent can be harmful to health. If you choose to use it, it’s best to use safe studio practices.

I would avoid turpentine unless your space is really well ventilated. If you have adequate ventilation then Gamblin’s OMS, oil of spike lavender or citrus solvent would be safer to use.

Safe studio practices when using solvent include:

• Containing it in a closed jar when it’s not in use
• Opening windows/ ventilating the room
• Wearing gloves when washing brushes with solvent
• Disposing of solvent and paint waste correctly

Don’t throw solvent away—it can be filtered and reused. Brush washers work to filter out paint sediment, leaving residue at the bottom and clean solvent to use in the container. 

If you don’t have a brush washer and are using a regular glass jar, leave the used thinner for a couple of days and wait for the sediment to settle at the bottom before cleaning it. Get a paper coffee filter and pour the dirty solvent through into a new jar. 

If you do need to eventually throw your solvent away, you’ll need to take it to your local hazardous waste facility. 

How to thin oil paint with solvent

Mixing into paint

When mixing solvent into paint, you really don’t need to use much to dilute the paint properly and get a runny consistency.

A few drops will be enough to get the desired effect when using oil of spike lavender. If you’re mixing lots of paint for a large piece, increase the quantity.

When using turpentine, you can be a little more liberal with the quantity.

Always mix extra oil medium with OMS when thinning.

Drying oils like linseed that are used as the binders in oil paint dry in a completely different way to solvent. Solvent dries by evaporation. While the oil undergoes a curing process to set. The oil becomes dry to the touch after about two days, but over the course of several months it oxidises to reach its final form. You won’t notice this oxidation process happening, but it’s important to know about, as it will inform your painting process and the way you layer your paint.

Follow the fat over lean rule. Thin paint (i.e. paint that has been mixed with solvent) should be layered first. Fat paint (paint that has more oil content) should be applied on top of lean layers. This guideline is perhaps the most essential technique to learn in oil painting. Thinned paint layers are brittle, so if they are layered on top of oil rich layers that are still curing (and therefore expanding and contracting slightly), they could dry first and then crack due to the movement of the layers beneath.

By applying paint in this order, you can ensure that you will get the best results in your work.

Cleaning brushes

To clean brushes with solvent, make sure you have a lidded container to keep the solvent in. Swirl your brush around in your brush washer to loosen the paint from your brush. Then, using a paper towel or lint free cloth, wipe the residue. It’s the friction that will remove the pigment from the bristles. Wear gloves when you do this in case any comes into contact with your hands. 

I would also advise getting a brush conditioner. The Master’s Brush soap conditions brushes, maintaining their shape and quality. Using solvent alone can leave brush fibres dry and brittle. The conditioner also cleans paint from brushes, so you can just stop using solvent all together if you wish. 

A safety tip: oil soaked paper towels or cloths should be disposed of properly. Drop a reusable, oily cloth into soapy water to get rid of the oil residue. Place used paper towels in an airtight container filled with water. When this is full take it to your local hazardous waste facility, or contact your local council to ask where to dispose of it properly. Do this instead of just throwing it into the trash, as drying oil can be combustible when oxidising. If you have a build up of oil soaked towels, it can be a fire hazard.

Do you have to use solvent to thin oil paint?

Oil painting solvent-free is really easy! Alternative mediums can be used to mix into paint to make it runnier and easier to handle. Mediums such as linseed oil, or Liquin fine detail will add to the body of the paint and reduce viscosity.

Cleaning up after oil painting without solvent is easy too. Just get some brush soap that will clean, condition and preserve oil paint brushes. I use the Master’s Brush Soap, as it keeps my brushes in perfect condition—springy and tapered to a point, which is what I need for detail work. 

As you can see, thinning oil paint might be more difficult than say watercolour, acrylic or water mixable oil, but once you have the right materials and get used to the process, it doesn’t feel like an obstacle. To keep your space free of heady fumes, read more detail in our solvent-free oil painting guide.

Read our studio safety tutorial to make sure your oil painting practice is as safe as possible.

Thinning oil paint: Pin it!

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