Can You Thin Polyurethane? (Water & Oil-Based)

Applying polyurethane can be a hard task because of the thickness of the sealant. However, thinning can make it easier to apply. So, can you thin polyurethane?

You can thin water-based polyurethane with water or denatured alcohol. While oil-based polyurethane can be thinned with turpentine, mineral spirits, or paint thinner. 

You have to use the right amount of paint thinner to prevent over-thinning. 

Thinning Ratio

If using a spray gun, dilute polyurethane in a ratio of 4:1 (4 parts polyurethane to 1 part of paint thinner). If using a paintbrush, dilute it in a ratio of 3:1. 

You must always use more polyurethane than thinning compound in the mixture. If you use more thinning compounds, you will over-thin the sealant. Another rule to go by is to dilute the sealant with 20% paint thinner (1/5).

Also, you must always dilute it more when using a spray gun than when using a paintbrush. That’s because the spray gun has a small nozzle, and the polyurethane has to be thin enough so it can be expelled easier. If you don’t thin it for a spray gun, you will clog the nozzle.

The right way to dilute it is to add the thinning compound to the polyurethane. This way, you can monitor the flow and consistency of the sealant to prevent over-thinning. However, if you add polyurethane to the paint thinner, you risk mixing too much or too little of it.

If you over thin polyurethane, the following things will happen:

  • It becomes too light to apply.
  • The color will be affected if the sealant is tinted.
  • It will run and drip, especially on vertical surfaces, such as walls, leading to a messy work area.
  • It will not stick properly to the surface.
  • The finish will become blurry.
  • It will not be as water-resistant as it should be.
  • It will not be durable and will come off within the first six months after application.

How To Thin Polyurethane?

Thinning polyurethane isn’t difficult, but the task requires caution, so you don’t over-thin it. 

Here are the tools you need:

  • A glass jar
  • Polyurethane
  • Paint thinner (water or mineral spirits)
  • Paint mixer or clean turning stick
  • A paintbrush
  • A pair of large scissors
  • Clean rags

1. Clean The Glass Jar

Clean The Glass Jar

You need to pour the polyurethane into a glass jar to dilute it, but the glass jar has to be clean. To clean the jar, use a rag to wipe off the stain. For tough stains, use warm soapy water or rubbing alcohol to dampen the rag.

Instead of the glass jar, you can also use a paint bucket. But, using a glass jar will help you keep track of the clarity of the sealant. With a glass jar, you can see the flow and how clear the poly is. You will also know when to stop thinning or stirring.

2. Open The Polyurethane Container

Open The Polyurethane Container

Polyurethane usually comes in spray cans, making it difficult to dilute it since the only way to get into the spray can is through the nozzle. So you need to cut the spray can to pour out the sealant.

To do this, make a dent in the paint can by using a nail or screwdriver. Then use a pair of scissors to cut the paint can. When you have a clean cut, pour the polyurethane into the clean glass jar. Or, if you have it in a bucket, just open the can and pour it into the clean glass jar. 

3. Measure and Add The Paint Thinner

Measure and Add The Paint Thinner

To dilute water-based polyurethane, use. To dilute oil-based polyurethane, use mineral spirits or turpentine.

Add the paint thinner in a ratio of 1:4 (one part paint thinner and four parts polyurethane). You can also use a measuring cup; add 1/2 cup of paint thinner to 1 gallon of polyurethane.

Use can use more or less paint thinner based on the chosen application method. To spray the polyurethane, the sealant should be as thin as possible. To apply it with a paintbrush, you don’t have to dilute it as much.

4. Stir The Mixture

Stir The Mixture

Next, mix the thinning compound and the polyurethane using a turning stix (or paint mixer) for 5-10 minutes. 

Don’t stir too fast or too hard, as that can cause bubbles in the coating. Instead, stir in a figure 8 motion, like you are trying to write “8” inside the mixture with the turning stick.

Once the sealant has an even flow and there’s no separation in the paint particles inside the glass jar, stop stirring. 

5. Test The Polyurethane

Test The Polyurethane

Next, test the thinned sealant on a cardboard or dispensable item. If it goes fine and the coating has no bumps or color variations, you are good to go. If the sealant is too thick or thin, add more polyurethane or thinning compound to the mixture. 

Products To Thin Polyurethane With

What Can You Use To Thin Polyurethane Paint?

You must thin oil-based polyurethane with mineral spirits and water-based polyurethane with water. Let’s check out some popular paint thinning compounds you can use:

Paint Thinner

You can use paint thinner to dilute polyurethane, but you must use the right paint thinner based on the sealant type. For oil-based polyurethane, use oil-based paint thinner. For water-based polyurethane, use water-based thinner. 

Water and oils don’t mix. So, if you use the wrong type of paint thinner, you’ll end up blurring the polyurethane coating. This will make it difficult for the clear coat to remain clear even after drying.

Lacquer Thinner

You can use lacquer thinner to thin oil-based polyurethane only. This is because lacquer thinner contains oily deposits, and it is used to thin lacquer, which is an oil-based sealant too. If you thin water-based polyurethane with lacquer thinner, you risk ruining the flow and clarity of the finish.

Denatured Alcohol

Denatured alcohol is better at thinning water-based polyurethane because the solvent can’t harm plastic. However, you must be careful while using it as you can over-thin the sealant easily. 


You shouldn’t use acetone to dilute polyurethane. That’s because acetone is a paint-removing solvent, which can separate the polymer from the binder. That makes it difficult for the sealant to stick properly.

Acetone can make the polyurethane finish dry too fast. This means the solvent will be evaporated too quickly and this can cause the finish to crack with time.

Related Read: Can You Tint Polyurethane Paint?

Things To Know

Dry Time

Thinned polyurethane does dry faster. Once its thickness has been diluted, it will have a lighter flow. Since there’s less sealant on the surface, the solvent will evaporate much faster, leading to a faster dry time. 


You must thin polyurethane before spraying it because the sealant is too thick to fit the sprayer’s nozzle. Thinning polyurethane for spraying also helps to archive a super smooth finish. When the sealant is thinned, it becomes lighter and can be sprayed evenly with no problems. 

However, if the sealant comes in a spray can, you don’t have to thin it. The polyurethane that comes in aerosol spray has been designed to be as thin as possible for easy application.

Should You Do it?

You should dilute polyurethane if its coating is too thick or if you are using a sprayer. If the coating is too thick, it will take longer to dry, also a thick flow can clog a sprayer. So, it’s best to thin it. 

Final Words

To thin oil-based polyurethane, use mineral spirits or turpentine. To thin water-based polyurethane, use water. If you mix the thinning compounds, you will ruin the sealant. Also, use a ratio of 1:4 if you are spraying it, and a ratio of 1:3 (1 part thinning compounds to 3 parts polyurethane) if you are using a brush to apply it. 

Tony Adams
Tony Adams

Tony is a professional painter and an author of DIY Geeks. Tony has completed over 1,000 painting projects for his clients. It's safe to say he knows what he Is talking about,

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