Can You Thin Polyurethane Paint? (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?

Yes, you can thin polyurethane to make it lighter or easier to apply. 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. So, how much is that? Let’s see. 

The Ratio To Thin Polyurethane

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

You must always use more polyurethane than paint thinner or mineral spirits. If you use more paint thinner than polyurethane, you will over-thin the polyurethane. Another rule to go by is to thin the polyurethane with 20% paint thinner. This means adding 1/5 of the paint thinner to the polyurethane.

Also, you must always thin polyurethane 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 fit so it can be expelled easier. If you don’t thin polyurethane for a spray gun, you will clog the spray gun’s nozzle.

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

What Happens If You Over-Thin Polyurethane?

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

  • The paint will become too light to apply.
  • The paint color will be affected if the polyurethane is tinted.
  • The polyurethane will run and drip, especially on vertical surfaces like walls leading to a messy work area.
  • The paint will not stick properly to the surface.
  • The polyurethane will not be as water-resistant as it should be.
  • The finish will become blurry.
  • The polyurethane 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 the polyurethane. To thin polyurethane, you’ll need the following tools and supplies:

  • A glass jar
  • Polyurethane
  • Paint thinner (water for water-based polyurethane and mineral spirits for oil-based polyurethane)
  • 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, but the glass jar has to be cleaned. To clean the jar, use a clean 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 polyurethane. 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 thin the paint 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 polyurethane.

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 polyurethane in a paint 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

Measure and add the thinning medium to the polyurethane.

To thin water-based polyurethane, the polyurethane thinner should be water. To thin oil-based polyurethane, use mineral spirits or turpentine.

Add the paint thinner in the 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 the polyurethane with a paintbrush, you don’t have to thin the polyurethane as much.

4. Stir The Mixture

Stir The Mixture

Once you have added the thinning compound to the polyurethane, you must stir the mixture to have an even flow. Stir for 5-10 minutes. 

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

Once polyurethane 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 polyurethane on cardboard or any dispensable object. To apply polyurethane, you can use a sprayer or a paint brush.

If the polyurethane goes fine on the material with no lumps, color variation, or imperfection, you have successfully thinned the polyurethane.

But, you must wait till the poly dries to check the finish if it’s good enough. If the paint feels too thin, you can add more polyurethane and then stir again. If the paint appears too thick, you can add more paint thinner and then stir again.

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 to see if you can thin polyurethane with them.

Paint Thinner

You can use paint thinner to thin polyurethane. But you must use the right paint thinner based on the type of polyurethane. 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 a water-based paint thinner to thin oil-based polyurethane, you’ll end up blurring the polyurethane coating. This will make it difficult for the clear coat to remain clear even after drying.


You can use water to thin water-based polyurethane only. That’s because water-based polyurethane uses water as the vehicle and carrier of the paint particles. As such, the polyurethane can be thinned with water without affecting the paint’s viscosity or flow.

You should never use water to thin oil-based polyurethane. Doing this will separate the oils from the water, and when the paint is applied, it will not be clear when dry. Rather, the polyurethane will be muddy, sticky, and gummed up.

Lacquer Thinner

You can use lacquer thinner to thin oil-based polyurethane only. This is because lacquer thinner contains oily deposits, and the substance is used to thin lacquer, which is an oil-based sealant. As such, the lacquer thinner can be used to thin oil-based polyurethane.

If you thin water-based polyurethane with lacquer thinner, you risk ruining the flow and clarity of the finish.

Also, you shouldn’t mix or shake the polyurethane container after mixing it with lacquer thinner to prevent paint bubbles from forming in the paint.

Lacquer thinner has a stronger odor than mineral spirits or any other paint thinner. So while thinning the polyurethane with lacquer thinner, prepare for the strong fumes that will be produced.

Denatured Alcohol

You can use denatured alcohol to thin polyurethane. But, the solvent is more ideal for thinning water-based polyurethane. That’s because alcohol is safe for polyurethane as the liquid can’t harm plastic. But care should be taken while thinning polyurethane with denatured alcohol.

Mineral Spirits

You can use mineral spirits to thin oil-based polyurethane only. This is because mineral spirits contain oily resins gotten from petroleum distillation and trees.

Since water doesn’t mix well with oil, using mineral spirits (which is oil-based) to thin water-based polyurethane (which contains water) will lead to the separation of paint particles. Also, the finish will not be clear. So only use mineral spirits to thin oil-based polyurethane.


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

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

If you thin polyurethane with acetone, you’ll get a polyurethane finish that doesn’t stick, dry well, or have an even flow.

Related Read: Can You Tint Polyurethane Paint?

FAQs About Thinning Polyurethane

Does Thinned Polyurethane Dry Faster?

Thinned polyurethane does dry faster. Once the thickness of the polyurethane has been diluted, the paint will have a lighter flow. As a result, the polyurethane solvent’s evaporation rate will be faster.

The reason polyurethane dries slowly is because of the thickness of the paint. But when the paint is thinned, it becomes lighter and as such, it will dry faster. The thin nature of the paint will speed up the rate of evaporation giving you a quick-dry polyurethane coating.

Thinned water-based polyurethane will dry faster than thinned oil-based polyurethane. That’s because water-based polyurethane has a lower volume of oils and resins that can prolong the paint’s dry time.

Should You Thin Polyurethane When Spraying?

If the polyurethane is too thick, you must thin it before spraying. Thinning polyurethane for spraying also helps to archive a super smooth finish. When the polyurethane is thinned, it becomes lighter and can be sprayed evenly with no problems. 

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

But, if the polyurethane comes in a paint bucket, you must thin it. If you don’t thin polyurethane before spraying, you can clog the paint sprayer nozzle, and the finish won’t come smooth. 

Should You Thin Polyurethane?

You should thin polyurethane if the poly coating is too thick. Also, if you are spraying polyurethane, you must thin it. 

If the polyurethane is too thick, it will take longer to dry. Also, thick polyurethane can clog a paint sprayer. So, thinning polyurethane before applying is recommended. 

Final Words

You have now learned that you can thin polyurethane, but the polyurethane thinner you use depends on the type of polyurethane. Use water to thin water-based polyurethane, and mineral spirits for oil-based polyurethane.

Also, ensure to measure the right amount of paint thinner so you don’t over-thin the polyurethane. As long as you thin the paint with the right amount and type of paint thinner, you’ll enjoy a lighter paint that dries faster and is easier to apply.

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