How to Thin Water Based Polyurethane?

Water-based polyurethane is becoming quite popular in the DIY space because of its non-yellowing formula which can be used over light-colored paints. I have used water-based polyurethane many times now on furniture makeovers when I use white paint. I like to spray my water-based poly to get the best possible finish, but its best sprayed once thinned.

The best way how to thin water-based polyurethane is to add 10% water into your overall volume. For example, this means if you have 1 quart of water-based polyurethane add 3.2 ounces of water (0.4 cups or 95 milliliters).

Why Thin Water Based Polyurethane?

The main reason you will need to thin water-based polyurethane is to use it in an HVLP spray gun. Spray guns typically require thinning for better spraying results, products such as oil-based polyurethane and lacquers would not use water as a thinner but instead mineral spirits or lacquer thinners. All water-based products, whether finishes or paints can be thinned using water, do not use paint thinners or mineral spirits or you will have a mess.

Feel free to get into more detail about spraying water-based poly in our “Can You Spray Polycrylic With A Spray Gun?” article.

Polycrylic is a water-based polyurethane created by Minwax, its the most common choice water-based poly on the market in my opinion. They sell it at most big box stores and it’s easily found on Amazon by clicking here.

Thinning water-based polyurethane when applying using a brush will allow you more time before it dries. This means your brush will glide easier and you won’t have that dry painting feeling that leaves you with a rough brushed surface. However, this is only to worry if you are painting in warm dry conditions, you may skip thinning and just have a spray bottle to use when you feel things drying.

Can I Thin Water Based Polyurethane with Floetrol?

Yes, you can thin water-based polyurethane using Floetrol however there is now a risk your water-based poly can turn yellow over time. Floetrol is designed for latex-based paints and is meant to improve flow when applying using a brush or roller. Since water-based polyurethane is already quite thin and flows quite easy it isn’t necessary when applying using a brush or roller.

If you are working in a warm work environment adding no more than 10% Floetrol to your water-based polyurethane could improve your flow during application. But water could also do the same effect, so in my opinion, I would stay away from Floetrol when using water-based polyurethane, it isn’t necessary.

How to Thin Water Based Polyurethane for Spraying?

To get the best results when spraying water-based polyurethane is to mix in 10% water. It is not recommended to go above 10% water, but depending on your spray gun test different amounts to get the best possible finish.

I use a Wagner Flexio 590 spray gun and mix my Polycrylic (water-based poly) at 10% of its volume and I get great finishing results. At first, I had issues when I didn’t thin it, but after testing different water percents my gun finally sprayed at a perfect consistency. Not all guns are the same, some require no thinning, and others require more. Testing water amounts has always been my best solution.

How to Thin Water Based Polyurethane for Wiping?

Adding 10-15% water into your water-based poly can help thin it to make wiping a potential option for application. But water-based polyurethane isn’t designed for wiping on using a rag, it’s best to apply with an applicator sponge for a smooth finish.

The Qualrite Paint Sponge Applicator is a great option to wipe on your water-based poly. When wiping water-based polyurethane don’t rub back and fourth, just do a once-over wipe. This does a quick thin coat layer and you can do this 3-4 times for a good durable finish.

Final Thoughts

All in all, thinning water-based polyurethane is simple, just use 10% water and you will be good to go. Remember you don’t always need to thin water-based poly, it’s recommended when you are spraying it or you are working in a warm and dry environment.