Can You Thin Polyurethane Paint? (How-To)

Applying polyurethane can be a bit of a daunting task due to the thickness of the paint. However, thinning the paint can make it easier to apply. But, can you thin polyurethane paint?

Yes, you can thin polyurethane paint either to make the paint lighter or easier to apply. But the thinning compound to be used will differ based on the type of polyurethane.

Water-based polyurethane should be thinned with water or denatured alcohol. While oil-based polyurethane should be thinned with turpentine, mineral spirits, or paint thinner.

You should also know that the polyurethane should be thinned with the right amount of paint thinner to prevent over-thinning.

How Much Can You Thin Polyurethane?

You should thin polyurethane in the ratio of 4:1 (4 parts polyurethane to 1 part paint thinner) if using a spray gun. Or, 3:1 (3 parts polyurethane to 1 part paint thinner) while using a paintbrush.

The polyurethane should always be more than the paint thinner or mineral spirits. 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.

If you want to spray the polyurethane, you need to get the paint lighter than when you want to use a paintbrush. This is because the polyurethane needs to be as light as possible while using a spray gun or paint sprayer so the polyurethane paint can be expelled from the nozzle easily.

You should know that 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.

If you add the polyurethane to the paint thinner, you risk mixing too much or too little polyurethane.

What Can You Use To Thin Polyurethane Paint?

What Can You Use To Thin Polyurethane Paint?

You should 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 should use the right paint thinner based on the type of polyurethane. You should use an oil-based paint thinner to thin oil-based polyurethane. And water-based paint thinner for water-based polyurethane.

This is because 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.

Acetone

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

Acetone can also 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 likely get polyurethane that doesn’t stick, dry well, or have an even flow.

Water

You can use water to thin polyurethane but this should only be done with water-based polyurethane. This is 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 the paint’s viscosity or flow being affected.

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 polyurethane but only use lacquer thinner to thin oil-based polyurethane. This is because lacquer thinner contains oily deposits and the substance is used primarily to thin lacquer which is an oil-based sealant. As such, the lacquer thinner can and should only 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. You shouldn’t mix or shake the polyurethane paint container after mixing it with lacquer thinner to prevent paint bubbles from forming in the paint.

You should also know that 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.

The main reason for this is that alcohol is safe to use on 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 polyurethane but the mineral spirit is more ideal for thinning oil-based poly. 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.

Now that you know what to use and what not to use to thin polyurethane, let’s check out how to thin polyurethane.

How To Thin Polyurethane (Made Easy)?

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

Here is a quick rundown of how to thin polyurethane:

  1. Clean the glass jar
  2. Open The Polyurethane Container
  3. Measure and add the thinning medium to the polyurethane
  4. Stir till you have even flow
  5. Test the polyurethane.

Now, let’s get to work.

1. Clean The Glass Jar

Clean The Glass Jar

The first step is to clean the glass jar thoroughly. This is to remove any paint or dirt in the glass jar. 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.

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 how clear the poly is and also the flow. 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 which makes 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, you first need to make a dent in the paint can. You can use a nail or screwdriver for this. Then use a pair of scissors to cut the paint can. When you have a clean-cut, pour out the polyurethane into the clean glass jar.

Or, if you have polyurethane in a paint bucket, just open the can.

3. Measure and Add The Paint Thinner

Measure and Add The Paint Thinner

The next step is to measure and add the thinning medium to the polyurethane. For water-based polyurethane, add water to the paint. For oil-based polyurethane, add mineral spirits or turpentine to the paint.

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

You can increase or limit the amount of paint thinner that you add to the polyurethane based on the chosen method of application.

To spray the polyurethane, you need the paint to be as light as possible so you might need more paint thinner. To apply the paint using a paintbrush, you don’t even need to thin but you can go with 1/4 cup of paint thinner.

4. Stir The Mixture

Stir The Mixture

The next step is to stir till you have an even flow. This will take between 5 minutes and 10 minutes of stirring.

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

When you notice that the polyurethane has an even flow and there is no separation of paint particles inside the glass jar, you can stop stirring.

5. Test The Polyurethane

Test The Polyurethane

The next thing to do is to test the polyurethane on cardboard or any dispensable object. You can spray the polyurethane or apply it with a paintbrush.

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

But you should 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.

Should You Thin Polyurethane When Spraying?

You don’t have to thin the polyurethane to spray it. But, doing so helps to achieve a super smooth finish. This is because when the paint is thinned, it becomes lighter and can be sprayed evenly with no problems.

Also, thinned polyurethane will come out smoothly from the nozzle making it possible to cover large surfaces quicker and better. However, you don’t have to thin the polyurethane to spray it, especially if the polyurethane comes in a spray can.

The polyurethane that comes in an aerosol spray can have already been designed to be as thin as possible for easy application. So you don’t have to thin.

But if the polyurethane doesn’t come in a spray can and you wish to spray it, you should thin the polyurethane before loading the paint into the spray gun or paint sprayer. This will ensure better application and a smooth result.

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 was 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 6 months after application.

Related Read: Can You Tint Polyurethane Paint?

Does Thinned Polyurethane Dry Faster?

Thinned polyurethane does dry faster. This is because the thickness of the polyurethane has been diluted and the paint now has a lighter flow. As a result, the rate of evaporation of the paint solvent and solidification of the paint particles 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 or less thick 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.

You should know that 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.

Final Words

Overall, you can thin polyurethane but the thinning compound to be used will depend solely 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.

Leave a Comment