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

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Polyurethane offers a multifaceted protective finish that effectively shields wood surfaces from water damage and scratches. For novice woodworkers, thinning polyurethane presents numerous benefits, enhancing the overall application process. 

But how do you thin polyurethane for spraying? Worry not, our woodworking pros are here to assist you every step of the way.

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. 

Ensure the jar is clean and free of dust, paint, or residues by washing it with soapy warm water and wiping it with alcohol to prevent interference during the thinning process.

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

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, we 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, we recommend purchasing 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

Avoid using acetone to thin polyurethane, as it separates the polymer from its binder, hindering proper adhesion to the wood. Acetone also causes polyurethane to dry too quickly, leading to cracks 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


It’s not a necessary step to thin polyurethane for spraying, but there are a lot of advantages in doing so. 

The desired consistency can vary depending on the specific polyurethane product and spray gun used, so adhering to a fixed ratio may not always yield the best outcome, and over-thinning remains a potential issue.

Robert Johnson is a woodworker who takes joy in sharing his passion for creating to the rest of the world. His brainchild, Sawinery, allowed him to do so as well as connect with other craftsmen. He has since built an enviable workshop for himself and an equally impressive online accomplishment: an extensive resource site serving old timers and novices alike.
Robert Johnson
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