How to Thin Water Based Polyurethane for Spraying?

Water-based polyurethane is one of the hottest products in the woodworking world, mainly because it provides a clear coat compared to the oil-based polyurethane that yellows over time.

While the yellow hue of oil-based polyurethane is still liked by some, there is a significant shift towards water-based polyurethane as it’s simply a product of newer technology with plenty of benefits.

If we are to list a few of these benefits, water-based poly is clear, less hazardous, dries faster, and is quite versatile compared to oil-based poly.

That being said, just like oil-based polyurethane, water-based polyurethane can require thinning before it’s ready for spraying, and due to the differences in chemical composition, the process for doing so is slightly different.

So, how to thin water-based polyurethane for spraying?

While the recommended ratio of polyurethane to water is often stated on the packaging, the best way to thin polyurethane is to add water in small increments until the desired consistency is reached, up to a point where water makes for 10 percent of the volume at most.

As the amount of water required can vary for different brands of water-based polyurethane, instantly using a 90 percent polyurethane to 10 percent water ratio can cause your poly to become way too thin.

Taking the process slow and testing at certain increments is the best way to ensure that you don’t run into trouble regarding the consistency of the polyurethane just as you are about to start spraying it.

How to Thin Water Based Polyurethane with Water?

If you never went through the process of thinning water-based polyurethane before or had issues with thinning, here are the steps we recommend taking to thin water-based polyurethane effortlessly.

  1. Start by transferring the amount of polyurethane you’re planning to use into a separate container. Remember that the total volume will be slightly bigger after the thinning process, so ensure that your container has enough space.
  2. Add an amount of water that will roughly translate to 2 to 3 percent of the total volume. For instance, if you have 16 fluid ounces of polyurethane in your container, add 0.3 to 0.5 fluid ounces of water.
  3. Slowly mix the combination of polyurethane and water with the help of a stirring stick. The key to a clean finish is to stir the poly gently without shaking it up too much.
  4. Transfer some of the polyurethane into your spray gun and see whether you are getting a consistent spray or not. If you are feeling comfortable with it, you can conclude the process here.
  5. If you didn’t achieve the consistency you are looking for, add another 2 to 3 percent of water, stir, and test again. You can repeat this process until you hit 10 percent of water, but you most likely will have reached the desired consistency by then.

Does Water-Based Polyurethane Need to Be Thinned to Spray?

In some cases, you don’t need to thin water-based polyurethane to be able to spray it.

Whether you will require to thin your polyurethane to spray it or not depends on your spray gun and your polyurethane, so you will have to find out for yourself whether thinning is necessary or not with a simple test.

Add a small amount of polyurethane into your spray gun without thinning it, and try spraying it on a surface. Even if you don’t have prior experience with spraying polyurethane, you can easily tell when poly is too thick as it won’t spray well.

Can You Thin Water Based Polyurethane with Floetrol?

Floetrol is a substance that is commonly used for thinning water-based (latex) paints and is known for producing better results than water in most cases.

So, what about using Floetrol for water-based polyurethane?

While you can indeed use Floetrol to thin your water-based polyurethane without issues, there is simply no reason to do so, as water works just fine.

As most water-based polyurethanes are produced in a way where they don’t even require any thinning at times, water does the job perfectly when thinning is required without the need for any extra products.

That being said, we have noticed that a few people swear by using Floetrol for thinning water-based polyurethane, meaning that the room for experimentation is always open if you are looking for alternatives that may improve your finish.

Can You Thin Water Based Polyurethane with Mineral Spirits?

As a rule of thumb, water-based finishes, in general, won’t work well with mineral spirits.

While mineral spirits are widely used to thin oil-based polyurethane, you should never use them to thin water-based polyurethane.

With water-based polyurethane easily being thinned with regular water that can be found in every household, there is no need to bring mineral spirits into the process either.

Can You Thin Water Based Polyurethane with Alcohol?

While denatured alcohol is used in certain areas of woodworking as a solvent, it’s not recommended to use it to thin water-based polyurethane for the same reasons that we don’t recommend using mineral spirits.

Simply put, water-based polyurethane doesn’t require anything more than water for thinning, and solvents such as mineral spirits and denatured alcohol don’t play well with water-based finishes.

How to Thin Water Based Polyurethane for Wiping?

Aside from a change in polyurethane to water ratio, the process for thinning water-based polyurethane for wiping remains the same as thinning it for spraying.

To thin water-based polyurethane for wiping, you should use a polyurethane-to-water ratio of 50:50, as wiping requires polyurethane to be quite thin for a good result.

That being said, you can also purchase polyurethane that is pre-mixed for wiping purposes under the name of wipe-on polyurethane.

Wrapping Up

When it comes to thinning water-based polyurethane for spraying, adding the water in small increments and testing the consistency as you do is the most important thing to do to ensure good results.

Since the consistency you get and the consistency you require depend largely on your polyurethane and your spray gun, following a set ratio often doesn’t yield the best results, with over-thinning being highly possible.