How To Thin Polyurethane For Spraying? (6 EASY Steps)

how to thin polyurethane for spraying

Polyurethane is a coating finish that is used to protect the wood from water damage and scratches. Plus, it improves the appearance of your woodwork by giving a glossy, smooth finish. You can apply polyurethane by brush or spray. When I was applying polyurethane by spray, I was curious about thinning polyurethane before spraying and wondered, How to thin polyurethane for spraying?

Here’s what I learned:

As a general rule, Oil-based polyurethane can be thinned with mineral spirit, lacquer thinner, or naphtha. Water-based polyurethane can be thinned with water or denatured alcohol. To prevent over-thinning, you should use polyurethane to mineral spirit ratio of 4 parts of polyurethane to one part of mineral spirit (4:1).

There’s a lot more to know about how to thin polyurethane for spraying to get a smooth thin coating as you wish.

In this article, we’re exploring the why thinning is important before spraying polyurethane and how to do that with both water-based and oil-based polyurethane.

Furthermore, I’ll answer some frequently asked questions about polyurethane thinning as well.

So, let’s jump in!

Do You Need to Thin Polyurethane To Spray?

Thinning polyurethane helps you to get an even, nice, and super smooth finish. But it’s not really a necessary thing.

Mostly polyurethane that comes in spray cans doesn’t have to be thinned, because they’re already designed to be as thin as possible. this will ease the application process. But if the polyurethane doesn’t come in a spray can and you’re willing to spray it, better to thin polyurethane first and then load it into the spray gun or sprayer.

Thinned polyurethane seals the wood way better and prevents from getting surface blotches and streaks. Thinned polyurethane gives less texture bumps and dries faster and more efficiently than regular polyurethane coatings. Naphtha, mineral spirit, and other solvents have a high evaporating rate. So, applying a thinner polyurethane will also dry faster because of the quick evaporation.

Rough regular polyurethane is difficult to dry, and the surface can be blotchy easily. If you’re a beginner in wood finishing stuff, better to work with thin polyurethane to minimize wood finishing issues.

Rough regular polyurethane has high moisture content. Therefore, drying is high and they lay much longer on the surface before drying.

Here’re the reasons why should thin polyurethane before spraying,

  • Thinned polyurethane dries faster
  • Thinned polyurethane minimizes wood finishing mistakes
  • They prevent the wood from getting a blotchy surface
  • Avoid getting tacky or sticky surface
  • Able to do the wood finishing quickly and efficiently

What Can You Use to Thin Polyurethane For Spraying?

Here’re the solvents you can use to thin polyurethane for spraying.

Oil-Based PolyurethaneWater-based Polyurethane
Mineral spiritWater
Oil-based paint thinnerWater-based paint thinner
Lacquer thinnerDenatured alcohol
Naphtha 

Never use Acetone to thin polyurethane for spraying. Acetone separates polymer from its biner which makes it difficult to stick to the wood properly.

Acetone dries out polyurethane too fast which causes polyurethane to crack with time.

So, let’s find out how to thin polyurethane for spraying. Start with thinning oil-based polyurethane.

How To Thin Oil-Based Polyurethane for Spraying?

To thin oil-based polyurethane, you need mineral spirit, naphtha, or lacquer thinner. They work as a solvent to thin out the polyurethane layer.

Here’re the requirements you need to have before thinning oil-based polyurethane for spraying.

  • Polyurethane
  • Mineral spirit or paint thinner
  • Glass jar
  • Clean stick for mixing
  • Clean rags
  • Spray gun or sprayer

Here’re the steps you should follow to thin oil-based polyurethane for spraying.

  1. Clean glass jar
  2. Pour polyurethane
  3. Add mineral spirit or paint thinner
  4. Stir the mixture
  5. Test the polyurethane
  6. Load polyurethane into the spray gun

So, let’s discuss each of the above steps in detail to get a clear idea about how to thin oil-based polyurethane for spraying.

1. Clean Glass Jar

The glass jar is better than paint buckets because you can easily see the reaction medium, and you’ll be able to mix polyurethane and mineral spirit incorrect ratio.

Cleaning a glass jar is very much important because it is the place where polyurethane meet mineral spirit and reaction happens. To thin polyurethane by mineral spirit, the reaction should take place in a clean environment without any interferences.

Clean and remove dust, and paints, in the glass jar using warm soapy water. You can remove stains and other residues by wiping them off using alcohol.

2. Pour Polyurethane

I don’t recommend you cut the spray can If you’re using polyurethane that comes in a spray can for spraying. Because spray cans are already designed with thin polyurethane and no need to thin them again.

The polyurethane that doesn’t come in a spray can are recommended to thin before spraying. So, open the polyurethane can and pour a sufficient amount of polyurethane into the glass jar.

3. Add Mineral Spirit or Paint Thinner

Then add mineral spirit or paint thinner to the glass jar. When you’re adding mineral spirits or paint thinner to the glass jar maintain the ratio of 1:4 meaning 4 parts of polyurethane to one part of the mineral spirit.

You can use a measuring cup to get the perfect ratio of polyurethane and mineral spirit. Pour ½ cup of mineral spirit into 1 gallon of polyurethane.

To spray polyurethane better use the polyurethane layer as light as possible. Therefore, adding paint thinner or mineral spirit a little more to the polyurethane can be accepted according to the type of the application.

4. Stir the Mixture

 Now take the clean stick and stir the polyurethane and mineral spirit mixture for about 10 minutes.

When you’re mixing, make sure to maintain even pressure on the stick and follow the figure 8 motion of stirring. Stirring too fast can cause bubbles in the polyurethane mixture. If the air bubbles are trapped inside, it can be a huge problem because they never leave even after the application of polyurethane coatings.

At the end of the stirring mineral spirit-polyurethane mixture, you won’t notice air bubbles. At that moment the thinning process of polyurethane has already been initiated.

5. Test the Polyurethane

After making thinned polyurethane test it on cardboard or a piece of wood with a spray gun. After the application test wait until polyurethane dries to see the true colors.

If the resulting polyurethane layer has no imperfections, no bubbles, or color variations, congratulations, you have successfully thinned the oil-based polyurethane.

  • If you feel polyurethane is too thin, add polyurethane a little more to the mixture, stir and do the test again.
  • If you notice polyurethane is still too rough, add mineral spirit a little more to the mixture, stir and do the test again.

6. Load Polyurethane into the Spray Gun

Once you are done with thinning polyurethane and the resulted mixture is thin enough, load to the spray gun or sprayer.

At this moment polyurethane is thin and you can achieve a super smooth glossy finish without making any blotches on the surface. It is so much easy to work with thinned polyurethane than the regular ones because they give excellent results.

That’s how you should thin oil-based polyurethane for spraying.

So, let’s find out how to thin water-based polyurethane for spraying.

How To Thin Water-Based Polyurethane for Spraying?

To thin water-based polyurethane, you need water or denatured alcohol. The amount of water required depends on the application and brands of a polyurethane product.

Here’re the requirements you should have to thin water-based polyurethane for spraying.

  • Polyurethane
  • Water or denatured alcohol
  • Glass jar
  • Clean stick for mixing
  • Clean rags
  • Spray gun or sprayer

Here’re the steps you should follow to thin oil-based polyurethane for spraying.

  1. Clean glass jar
  2. Pour polyurethane
  3. Add mineral spirit or paint thinner
  4. Stir the mixture
  5. Test the polyurethane
  6. Load polyurethane into the spray gun

So, let’s discuss each of the above steps in detail to get a clear idea about how to thin water-based polyurethane for spraying.

1. Clean Glass Jar

Follow the same procedure as we discussed in thinning oil-based polyurethane for spraying for Jar cleaning.

The cleaning glass jar is very much important because this is where the reaction between water-based polyurethane and water takes place and a clean environment without any interferences is a must.

Clean and remove dust, and paints, in a glass jar using warm soapy water.

2. Pour Polyurethane

Water-based polyurethane that doesn’t come in a spray can are recommended to thin before spraying. So, open the polyurethane can and pour a sufficient amount of polyurethane into the glass jar.

Make sure to take the glass jar that has enough space to contain polyurethane and water.

3. Add Water or Denatured Alcohol

Add water in small increments until it reached the desired amount of thin water-based polyurethane. Add water or denatured alcohol 2% – 3% percent of the total volume. For example, add 0.4 fluid ounces of water to the 20 fluid ounces of polyurethane volume.

Mostly, the amount of water you should add to the polyurethane has been stated on the polyurethane packaging.

To spray polyurethane better use the polyurethane layer as light as possible. Therefore, never add 90% of polyurethane to 10% of water instantly which can cause polyurethane too thin.

Adding water slowly while increasing the amount little by little until it meets the best volume to thin polyurethane is the best method to practice.

4. Stir the Mixture

 Now take the clean stick and slowly stir the polyurethane and water mixture for about 10 minutes.

Never move or stir too fast when stirring. Stirring too fast can cause bubbles in the polyurethane mixture. Therefore, make sure not to shake the mixture too much when stirring.

5. Test the Polyurethane

After making thinned polyurethane test it on cardboard or a piece of wood by adding a small amount of polyurethane to the spray gun. After the application test wait until polyurethane dries to see the true colors.

If the resulting polyurethane layer has consistent spray with no imperfections, no bubbles, or color variations, that indicate, you have successfully thinned the water-based polyurethane.

  • If you feel polyurethane is too thin, add polyurethane a little more to the mixture, stir and do the test again.
  • If you notice polyurethane is still too rough, add water or denatured alcohol a little more to the mixture, stir and do the test again.

6. Load Polyurethane into the Spray Gun

Once you are done with thinning water-based polyurethane and the resulted mixture is consistent and thin enough, load the mixture to the spray gun or sprayer.

To achieve a super smooth surface, you can add a maximum amount of water up to 10% by eventually increasing the water volume by 2% -3%.

So, that’s how you should thin water-based polyurethane for spraying.

Read: 15 Proven Methods To Fix Polyurethane Mistakes

Let’s answer some frequently asked questions regarding how to thin polyurethane for spraying.

How Much Should You Thin Polyurethane for Spraying?

  • Oil-based polyurethane should thin by adding 4 parts of polyurethane to one part of the mineral spirit. That basically means you should use oil-based polyurethane to mineral spirit ratio of 4:1
  • Water-based polyurethane should thin by adding water in small increments of 2% -3% until it reaches the optimum consistency and increases the adding water amount maximum up to 10% of the total volume. Polyurethane to water ratio differs according to the brands and application type.

What Happens If You Over-Thin Polyurethane for Spraying?

Over-thinned polyurethane never provides what real polyurethane gives to your woodworks or furniture. There must be a sufficient amount of polymer and binder to stick well to the wood while giving protection to the surface.

Over-thin polyurethane gives the following issues when applied.

  • The polymer coat will be too light in color
  • Results surface blotchiness after application
  • Polyurethane will not stick well to the surface
  • Doesn’t provide enough protection against water and scratches
  • Results blurry finish
  • Poor durability

Did I cover all you wanted to know about: How to Thin Polyurethane For Spraying?

In this article, we have deeply discussed How to thin polyurethane for spraying and why do you need to thin polyurethane to spray.

Thinning polyurethane e before spraying is not really a necessary thing to do. But by doing that, I’m 100% sure you’ll get a super smooth even finish than fining with regular rough polyurethane. Plus, if you’re a beginner in woodworking, I highly recommend o thin polyurethane for spraying to avoid wood facing most of the issues.

We have talked about how to thin both oil-based and water-based polyurethane with different types of solvents and answered some frequently asked questions as well. Hope you have gained good knowledge about how to thin polyurethane for spraying. Make sure to practice the steps you have learned in your next wood finishing project. Keep practicing!


About Walter Parker

Walter Parker is a father and woodworking enthusiast. He is passionate about woodworking projects & plays with woodworking tools having spent over 2 decades as a leader for Woodworking Planet. He wants to make people love woodworking!

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