bubbles in water-based polyurethane

How to Apply Water Based Polyurethane Without Bubbles

In Tips by Jamie P.

It all comes down to the final coat of water-based polyurethane, you are excited to finish up your amazing project to show off to your friends and family. But, when you come back to check out the final product after applying the topcoat there are bubbles, what did you do wrong? How should you apply water-based polyurethane without it having bubbles in the finish?

To avoid bubbles when applying water-based polyurethane it’s best to apply multiple thin coats using a synthetic brush rather than a natural brush. Dampen the entire brush with water to make the application even smoother. Lastly, do not shake up the can as it creates bubbles, stir the poly. 

What Causes Bubbles in Water-Based Polyurethane?

Shaking the Can

The first thing you need to realize is a can of water-based polyurethane is not like spray paint, DO NOT SHAKE IT. Shaking is not recommended for any woodworking or painting products, I always recommend stirring to prevent any bubbles. Shaking things up will cause bubbles to form on the top of the product’s surface, this is where your brush or applicator will dip in and prick these bubbles up, then they are drying on your project.

So keep this small tip in mind or there will be big consequences.

Air In the Brush

You might wonder and ask yourself “where are these air bubbles even coming from”. The answer is quite simple, there are air pockets in your brush causing these tiny bubbles. It is very important that you wet your brush before using almost any product. I wet my brushes with water under the sink, I don’t mind soaking them, but make sure you use your hand to remove as much dripping water before using it. Basically, you want all the bristles wet, this removes air in the brush and your bristles stick together making a nice gliding feel as you use it.

It is pretty simple, with oil-based products you dampen the brush with a solvent-based product, so with water-based we just use water. It is extremely important that you do not use solvent-based with water or you will have some issues, more than just bubbles in your finish.

Contaminated Surface

Although using a water-based polyurethane you shouldn’t have to many issues with contaminated surfaces, however, that doesn’t mean the water-based products won’t react funny to a tabletop. When I am prepping a project I like to either sand down to bare wood or strip it down. It can be a good idea to do a deep clean in the wood with mineral spirits when finished. Mineral spirits are a solvent-based product, but it cleans surfaces extremely well, and the best thing is it evaporates quickly so you can carry on.

One thing you must make sure of when using mineral spirits to clean a surface, let it completely dry before applying your water-based polyurethane. After it has dried, your surface should be quite clean, add your stain or whatever products you would like to use.

Use Shellac!

If you have had issues with bubbles and do not want to risk it now, USE SHELLAC first. Instead of hoping there’s no contamination just apply a couple of thin coats of shellac. Shellac is great because it will seal anything underneath it from causing issues above. This means you apply the shellac and once dry you can apply water-based poly right over it. Thats why I use shellac to stop bleed-through.

What’s the Best Way to Apply Water-Based Polyurethane?

In my experience, there are 4 ways you can apply water-based polyurethane which is by using a paintbrush, foam applicator, wiping it on, and using a spray gun. All options can be executed professionally, but I do have a preference. I have had the most luck spraying water-based polyurethane with my HomeRight Super Finish Max or my Wagner Flexio 590.

The spray guns I use are called HVLP spray guns, which means High Volume Low Pressure. Other spray methods exist, such as airless spray guns and also setting up HVLP with an air compressor. These spray guns I use are affordable and still produce a professional finish, I really like them! You need to properly thin the water-based poly before spraying or you will have issues, that’s why I made a guide and explain it in detail in my HomeRight Youtube Video.

Does Spraying Water-Based Polyurethane Leave Bubbles?

I never have had bubbles when spraying water-based poly, I only had issues when using a brush. The best thing about spraying you also have no brush streaks, it is a no-brainer for me when I want to sell refinished furniture for top dollar. Using a sprayer requires a proper area to spray because it leaves quite the mess if I am being honest. That would be an advantage to applying by brush, applicator, or wiping it on.

Water Down Your Polyurethane

A great way to prevent bubbles in your water-based polyurethane is to add water, but keep in mind there is a max amount you can put in without causing issues to the durability of the product. It is not recommended to thin water-based poly more than 10%, I aim to go just under the 10 mark and it works great in my sprayer. But, you can and should still thin the poly if you are having bubble issues, when applying you will notice it glides on easier, meaning a nice thin coat is being applied.

Bubbles will come from too much product, this is why applying multiple thin coats is important. A brush is probably the most difficult when trying to apply thin, it’s just hard to dip a brush and lightly apply the poly. That is why spraying is best, you can spray the thinnest coat and no bubbles have the chance to form, and if they do you are doing something wrong. A foam applicator can apply nice and thin too, but this takes some practice if you don’t want streaks, especially in dark paints.

How to Fix Polyurethane Bubbles That Have Already Dried?

Ok, it’s too late. You have bubbles in your finish and you are likely crying like a baby. Don’t worry, I am here to help! There are a few methods that can fix this problem, some more time-consuming and work-consuming than others.

Use Steel Wool

Steel wool can really do some magic, it’s best to start with 0000 and try to rub out th0e dry bubbles. The reason I recommend steel wool is because of how fine it is. You won’t see scratches, it will more so make it glossier, you might have to buff out the full surface to match if you aren’t adding another coat. If 0000 isn’t doing the trick try going down the line to 000 or 00 steel wool.

Sanding Smooth

You can always try sanding the surface with high grit sandpaper such as 400 or more. This will certainly polish off your piece but it can sand out those bubbles you have on your surface. You can directly sand the bubble area or sand everywhere to almost remove that layer. You can try an electric sander such as a random orbital sander, no matter what you do sanding can be risky here.

The best way to approach this by sanding is using a high grit sanding sponge, at least 220 but higher is better to start. Using the washable sponges will do less damage to your surface, I am not saying wet them but avoid using the cheap sponges that feel like a rock. These will just wreck your surface, so the softer the better.

Worst Comes to Worst, Restart the Surface

If your bubbles are that bad and sanding or steel wool isn’t doing the trick you may need to restart that surface. I have had issues before, what I did was protected the surfaces that were ok and use stripper on the damaged surface. I would paint or stain the affected area and then apply my water-based poly once again. This is time-consuming and hurts the soul, but sometimes you have no choice.

Final Thoughts

What have we learned? Well, a few things I hope, but this article has completely broken down ways to prevent and fix bubbles in water-based polyurethane. Poly is a product that can add protection or completely cause your frustration from one tiny mistake, it’s rough.

Knowing ways to prevent, like adding some water or applying poly in a different method, is all part of the learning experience. Woodworking, furniture painting, and all similar hobbies take time and experience, just enjoy the ride.