How To Mix Mineral Spirits With Polyurethane

Getting a beautiful and clean finish on a DIY project often involves polyurethane. In some cases, thinning this synthetic resin is necessary for optimal results, and mineral spirits are a suitable solvent for the job. So, how do you mix spirits with polyurethane?

You can mix mineral spirits with polyurethane by measuring the proper amounts of each and combining them in a separate container. The amount of mineral spirits you add to polyurethane depends on how thick you want the coating to be.

Keep reading for all the information you need to mix mineral spirits and polyurethane.

About Polyurethane and Mineral Spirits

Polyurethane and mineral spirits are used in various industries as well as by DIY’ers. Do-it-yourself projects of all kinds can benefit immensely from a mixture of spirits and polyurethane. Let’s have a closer look at both.


Polyurethane is an old invention, created about 90 years ago in the 1930s

This class of polymers is used in various applications such as insulation, car parts, and shoe soles. DIY’ers benefit immensely from polyurethane with varnishes and coatings. It’s especially beneficial when working with wood.

A coat of polyurethane finish provides durability and makes the wood resistant to water damage. This oil-based, smooth finish can also make wood surface staining stand out more than it would on its own.

Mineral Spirits

Mineral spirits are an oil-based liquid used as a solvent in painting, degreasing parts, and cleaning, among other industrial purposes. This solvent is popular because it is less toxic than others and is relatively cheap.

DIY’ers use spirits as paint thinner and to clean tools. Mechanics can get tough grease and grime off small and large car parts with spirits.

Why Mix Mineral Spirits with Polyurethane?

Thinning polyurethane means that you will have to apply more layers. A lot of DIY’ers thin their polyurethane precisely for this reason.

Oil-based polyurethane is mixed with mineral spirits to make a thinner coat and give you more control over the final coat. The added solvent also helps to eliminate air bubbles and wrinkles.

Sometimes, people assume that adding spirits to polyurethane will make the finish dry quicker, but this is not the case. The spirits will dry faster than the polyurethane, which will not be affected at all in this respect.

It is worth noting that the humidity and heat in your area can affect your finish. Thinning polyurethane with oil-based solvent is an effective solution for this.

How to Mix Mineral Spirits with Polyurethane

First, make sure you use oil-based polyurethane. Since mineral spirits are oil-based, thinning water-based polyurethane with it isn’t possible. Avoid a water-based finish when using oil-based solvents.

The labeling on the container will tell you what type of polyurethane you have. If you are dealing with polyurethane that is not in its original container and not labeled, you can figure out which type it is by its color and appearance. Oil-based polyurethane is semi-clear with an amber tint, while water-based polyurethane is white and milky.

Follow these steps to mix polyurethane with spirits:

1.      Get your materials together: polyurethane, mineral spirits, a glass or plastic container, and a wooden stirrer.

2.      Put on protective gear and work in a room with adequate ventilation or outside.

3.      Open the polyurethane container and stir it gently. The components will have settled at the bottom, and you want an even mix for good results.

paint stirrer

4.     Add 1 part solvent to 3 or 4 parts of polyurethane. Pour the required amount of polyurethane into a glass jar. 

5.      Pour the mineral spirits into the polyurethane.

6.      Stir the mixture. It is essential to stir slowly and not shake. Shaking will introduce air bubbles which will give you an uneven coat. Plus, avoiding air bubbles is one of the reasons to add spirits to polyurethane in the first place.

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How Much Mineral Spirits Should I Add to Thin Polyurethane?

Generally, it’s best to add 1 part solvent per 3 to 4 parts polyurethane. This is equivalent to 20 to 25 percent mineral spirits. If you want a thicker mixture, use the 1 to 3 ratio. For a thinner mixture, use the 1 to 4 ratio.

Safety: Mixing Polyurethane With Mineral Spirits

Polyurethane contains VOCs (volatile organic compounds) which makes it toxic. Overexposure can induce dizziness, headaches, nausea, nasal irritation, and asthma attacks. Acute exposure over a long time can lead to liver and kidney damage.

Mineral spirits are an irritant and less hazardous than polyurethane. Still, this solvent can cause skin burns as well as irritation to your eyes, nose, and throat.  

Polyurethane and spirits are flammable with a flashpoint of 105 degrees Fahrenheit.

This does not mean that polyurethane and spirits are unsafe to work with. The key is to employ proper safety measures when working with these materials. Consider the following safety tips closely:

· Wear protective gear

Use rubber gloves, goggles, and protective clothing when mixing and using polyurethane and spirits.

paint gloves

· Work outside or in a well-ventilated area

When working with toxic liquids, it is best to do so either outside or in a well-ventilated room. You might also consider wearing a respirator.

· Stay far away from heat sources

Since both polyurethane and spirits are flammable, keep them away from any heating sources and sparks. Do not smoke around these liquids and do not store them close to a radiator.

If you get any polyurethane or solvent on your hands or any part of your body while mixing or using them, wash the area with soap and water immediately.

Can I Use Polyurethane Without Mineral Spirits?

Yes, you can apply polyurethane without mixing it with mineral spirits. However, there is a high likelihood that after the initial coat, bubbles will form as you apply subsequent coats of polyurethane.

Thinning your polyurethane with spirits ensures that you do not have to deal with bubbles. As mentioned above, you can add one part mineral spirits to 3 or 4 parts of polyurethane to get the desired results.

Conclusion – Mixing Polyurethane with Mineral Spirits

Mixing polyurethane and mineral spirits is easy. Consider the amount of solvent you add to your polyurethane carefully to get the beautiful finish you are looking for. And always keep safety your #1 priority!