Can You Thin Polyurethane With Paint Thinner?

Paint Thinner for Mixing with Polyurethane

Do you spend time thinking about using paint thinner to thin polyurethane? Well, if you do, you’re not alone, for most beginners in finishing wood often ask this question. Of course, the answer to this query is a resounding, “Yes, you can!” If the polyurethane becomes too thick, you can use paint thinner to thin it out. Nevertheless, paint thinners come in a wide variety of ingredients. Many of them may contain various toxic and odorous substances. Hence, when selecting a paint thinner for thinning polyurethane, check the thinner’s label to ensure it is appropriate for thinning polyurethane.

You can use paint thinner to thin polyurethane and achieve the right consistency of the paint. Of course, there are better ways to thin polyurethane, yet it can do the job as well, given that many house owners have a stored paint thinner at home. Since paint thinner is readily available, if you need to thin polyurethane, you can always use paint thinner. But as mentioned above, ensure the thinner you will use is compatible with the polyurethane you use. 

How to Thin Polyurethane with Paint Thinner Correctly

You want your paint to be smooth and easy to use. As long as the paint thinner complements the polyurethane, you can always use it to thin the latter. Add thinner to the polyurethane but do it in stages to ensure it will stay sufficiently thick. 

There may be different thinning agents at hand that you can use to thin polyurethane, but you need to employ the correct thinning technique to ensure you can get the right consistency for your paint. So, when adding thinner, take extra care. You can best start by preparing a stirring stick. You can use any stirring stick as long as it is clean. 

Shaking the paint can is not recommended because it will only build up air bubbles in the paint, leading to a defective finish. 

Once you have a stirring stick, slowly add paint thinner, and do it in different batches. Use your stirring stick to stir the mix to get excellent paint consistency. Move the stirring stick in a figure-eight path to ensure everything gets mixed well. Remember to scrape thick material from the can’s sides and bottom. Go on stirring until you see a uniform consistency. 

Find the perfect consistency and texture for the paint. If the paint is still thick, continue to add thinner in a small amount. Do the pouring of thinner incrementally to ensure you will do it correctly. This way, you will make the polyurethane less watery in consistency. To ensure ideal consistency, you can employ the 1:4 ratio between polyurethane and thinner. 

Factors You Need to Know When Thinning Polyurethane

Aside from knowing the simple steps on how to thin polyurethane using paint thinner, it will also help if you are familiar with the following factors to consider when thinning polyurethane using paint thinner:

Thinning Ratio

Using a spray gun to apply polyurethane, you should dilute the polyurethane using the 4:1 ratio. This ratio means you will need to add one-part thinner for every four-parts polyurethane. However, if you use a paintbrush, you can follow the 3:1 ratio.

Remember to dilute the polyurethane more when using a spray gun. The reason is spray gun comes with a small nozzle. Thus, it will be best to thin the polyurethane more to avoid clogging the spray gun nozzle and allow the paint to ooze out effortlessly. 

The proper way of diluting polyurethane is by adding the thinner to the polyurethane and not vice versa. This way, you can control the consistency and the amount of thinner you add to the paint. 

If you do otherwise, you can avoid mixing too little or too many components. Remember that the polyurethane might appear light if you over-add thinner to the paint. Besides, the color of the polyurethane becomes lighter, too, if it is tinted. 

Moreover, it will drip and run, mainly if you apply it on an incline or vertical surfaces like fences and walls. Furthermore, it will stick to the surface poorly.

You may also end up with a blurry finish that lacks water-resistant properties. Thus, the finish becomes less durable and peels off or comes off with ease over time. 

Test Your Polyurethane after Thinning

After adding thinner to the polyurethane, you can test the mixture to see its consistency. If it shows inordinate thickness, you can add thinner. You can use a dispensable item or cardboard to test its property. If it goes well with the test material, you can apply it to the area you would like to paint. 

Drying Time

If you thin polyurethane, it exhibits faster drying properties. Since it contains less sealant, the solvent content it has will evaporate faster, resulting in a quicker drying time for the paint. Once you’ve diluted the polyurethane, it will flow lighter.


If you spray the polyurethane, you must thin it before you apply it. Its sealant might be too thick to squeeze itself out of the nozzle. But if you thin it, you can spray it easily, leading to a smooth finish. Likewise, the sealant becomes lighter, and you’ll have no issue when spraying it. 

Yet, if the sealant comes in a spray can, you don’t need to apply thinner. Remember that canned polyurethane has a consistency designed for easy application without needing thinning. 


When thinning polyurethane, you will need several tools like a glass jar, polyurethane, thinner, stirring stick, paintbrush, scissors, and clean rags. So, before applying paint thinner to polyurethane, ensure you have prepared all these materials and tools. 

The trick is simple—you don’t need to over-add or add a little thinner. It would be best if you achieved the proper consistency and balance to make the paint application smooth and easy. Of course, you can get the hang of the thinning process with much practice. 

Leave a Comment