How to Thin Polyurethane for Spraying — Water & Oil-Based Poly

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Polyurethane provides a versatile protective layer, safeguarding wooden surfaces from water harm and abrasions. If you’re relatively new to woodworking, you’ll find that thinning polyurethane can significantly improve the ease of application.

Wondering how to properly thin polyurethane for spraying? Rest assured, I’m ready to guide you through each stage of the process.

Is Thinning Polyurethane Required Before Spraying?

Thinning polyurethane can result in an even, attractive, and ultra-smooth finish, although it is not always necessary. Spray can polyurethane is usually pre-thinned for optimal application.  

However, if using non-aerosol polyurethane, thinning it before loading it into a spray gun or sprayer is beneficial. 

Minwax Fast Drying Polyurethane and brush

Thinned polyurethane offers better wood sealing, preventing surface imperfections like streaks and blotches. It also yields fewer texture irregularities and dries more quickly due to the rapid evaporation of solvents like naphtha and mineral spirits. 

Below are some reasons why thinning polyurethane before spraying is necessary:

polyurethane application

Thinning Water and Oil-Based Poly for Spraying: Required Tools & Steps

For thinning oil-based polyurethane, consider using solvents such as mineral spirits, lacquer thinner, or naphtha. For water-based polyurethane, water or denatured alcohol is necessary. 

The specific polyurethane product’s brand and application will dictate the required amount of thinning agent.

More about polyurethane: 

mixing polyurethane for air sprayer

Before thinning oil-based polyurethane for spraying, make sure you have the following:

Step #1: Clean Your Glass Jar

This is preferable to a paint bucket for thinning polyurethane, as it allows to facilitate observation of the reaction medium easily and accurate mixing of polyurethane with mineral spirits or water. 

To prevent any interference during the thinning process, it is important to make sure the jar is thoroughly cleaned and devoid of dust, paint, or residues. This can be achieved by washing the jar with warm, soapy water and then wiping it with alcohol to ensure its cleanliness.

Step #2: Pour the Poly (Water and Oil-Based)

In fact, experts advise against thinning polyurethane from spray cans, as they are already formulated with the optimal viscosity for spraying. 

materials for tinting polyurethane

However, for non-aerosol polyurethane products, thinning is recommended before spraying. Dispense an adequate quantity of polyurethane from the can into a clean glass jar for the thinning process.

Step #3: Add Paint Thinner or Mineral Spirit

Introduce mineral spirits or paint thinner into the glass jar, maintaining a 1:4 ratio (one part solvent to four parts polyurethane). For accurate measurements, utilize a measuring cup; for instance, mix ½ cup of mineral spirits with 1 gallon of polyurethane.

For optimal spray application, keep the polyurethane layer as light as possible. Slightly adjusting the amount of solvent following the specific application may be acceptable.

Step #4: Combine the Mixture

Stir the mineral spirit and polyurethane blend for around 10 minutes using a clean stick, applying even pressure and following a figure-8 stirring motion. Avoid stirring too quickly, as it may cause bubble formation in the mixture. 

mixing polyurethane and tint

Trapped air bubbles can create issues after the polyurethane coating is applied. The thinning process is considered complete once there are no visible air bubbles in the mixture.

Step #5: Test First

Apply the thinned polyurethane to a piece of cardboard or scrap wood using a spray gun [1] and allow it to dry to evaluate the final look. If the coat is devoid of flaws, bubbles, or color discrepancies, the oil-based polyurethane has been thinned effectively. 

Should the layer appear too thin, incorporate more polyurethane into the mixture, stir, and reassess. If the surface seems too coarse, I suggest adding additional mineral spirits, mixing, and retesting.

Step #6: Fill Your Spray Gun With Polyurethane

spraying polyurethane to cabinet doors

After successfully thinning the polyurethane to the desired consistency, load it inside the sprayer or spray gun. The thinned polyurethane allows for a remarkably glossy and smooth finish without surface blotches. 

Manipulating thinned polyurethane is more convenient compared to the non-thinned counterparts, as it consistently delivers exceptional results.

using paint roller to apply polyurethane

How Should You Thin Water-Based Polyurethane with Water?

To thin water-based polyurethane effortlessly, follow these steps:

  1. Transfer the polyurethane you plan to use into a separate container, ensuring extra space for the increased volume after thinning.
  2. Add water equal to 2-3% of the total volume (e.g., 0.3-0.5 fluid ounces of water for 16 fluid ounces of polyurethane).
  3. Gently mix the polyurethane and water using a stirring stick, avoiding excessive agitation.
  4. Transfer some of the mixtures into your spray gun and assess the spray consistency. If satisfactory, the process is complete.
  5. If the desired consistency is not achieved, add another 2-3% of water, stir, and test again. Repeat until reaching a maximum of 10% water, although the ideal consistency will likely be attained before this point.
applying Varathane 200241H Water-Based Ultimate Polyurethane to table

How Should You Thin Water-Based Poly for Wiping?

The process for thinning water-based polyurethane for wiping is similar to that for spraying, with the main difference being the polyurethane-to-water ratio. 

For wiping, use a 50:50 ratio to achieve a thinner consistency, which is crucial for optimal results. Alternatively, you may purchase a pre-mixed wipe-on polyurethane specifically formulated for wiping applications.

What to Use to Thin Polyurethane Before Spraying?

These solvents can be used to dilute polyurethane before spraying.

Water-based Polyurethane

Oil-Based Polyurethane

Oil Based Polyurethane on a wooden board

Steer clear of acetone when it comes to thinning polyurethane; it disrupts the balance by separating the polymer and its binder, thus impeding the finish’s ability to properly adhere to the wood surface. 

Additionally, acetone hastens the drying process of polyurethane, which might result in cracking and compromise the longevity of your finish over time.

Can You Use Floetrol to Thin Water-Based Polyurethane?

Floetrol is commonly used to thin water-based (latex) paints and often yields better results than water. While Floetrol can be used to thin water-based polyurethane without issues, it is often unnecessary, as water typically suffices. 

Many water-based polyurethanes are formulated to not require thinning, and water effectively serves this purpose when needed. 

However, some individuals prefer using Floetrol for thinning water-based polyurethane, so experimenting with alternatives to potentially improve the finish is always an option.

Floetrol sample

Can You Thin Water-Based Poly with Mineral Spirits?

As a general guideline, water-based finishes are not compatible with mineral spirits. Although mineral spirits are commonly used for thinning oil-based polyurethane, they should never be used to thin water-based polyurethane. 

Since water-based polyurethane can be easily thinned with readily available water, there is no need to introduce mineral spirits into the process.

polyurethane water

Can You Thin Water-Based Polyurethane Using Alcohol?

Although denatured alcohol serves as a solvent in some woodworking applications, it is not recommended for thinning water-based polyurethane, similar to mineral spirits. 

Water-based polyurethane only requires water for thinning, and solvents like mineral spirits and denatured alcohol are not compatible with water-based finishes.

How Much Should You Thin Poly for Spraying?

To dilute oil-based polyurethane, use a 4:1 ratio by mixing four parts polyurethane with one part mineral spirit. 

When working with water-based polyurethane, slowly add water in increments of 2%-3% until you reach the desired consistency, making sure not to exceed 10% of the total volume. 

Keep in mind that the ideal ratio of polyurethane to water may vary based on the brand and application type. 

spray painting chair with tinted polyurethane

What Will Happen If You Over-Thin Polyurethane?

Excessively thinned polyurethane cannot deliver the expected protection and finish for your woodworking or furniture projects. Adequate amounts of binder and polymer are necessary for proper adhesion and surface protection.

Over-thinned polyurethane can cause the following problems:

painting wooden furniture with Varathane 200241H Water-Based Ultimate Polyurethane

Conclusion

Thinning polyurethane for spraying isn’t always obligatory, but it does come with several advantages. The ideal consistency can vary depending on the particular polyurethane product and the type of spray gun in use. 

Therefore, adhering rigidly to a set ratio might not always yield the best results, and over-thinning is a potential concern to be mindful of.

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Robert Johnson is a passionate furniture maker & carpenter, sought after for his knowledge on the craft.
You’ve probably seen his down-to-earth wisdom in USA Today, Bobvila, Family Handyman, and The Spruce, where he has shared commentary and guidance on various woodworking topics.

Robert is the brain behind Sawinery, where he aims to share tips, tricks, and a passion for all things carpentry.

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