August 10, 2021

How to Apply Polyurethane without Bubbles

To apply polyurethane without bubbles, avoid using a natural brush as you apply polyurethane. Use a synthetic brush for bubbles. Synthetic brushes have bristles made with synthetic material that gives better application. 

They come in a wide range of sizes as they match various projects, from small to big projects. 

Synthetic brushes do not absorb polyurethane during use. Use water on the polyurethane brush to dampen it for a smooth and flawless application. Do not shake up the can since that creates bubbles. Rather stir the polyurethane.

What causes bubbles

Shaking the gallon

When you want to take your juice, it is thicker at the bottom hence the need to shake first. The polyurethane mixture comes in that form before use since it has been in the shop for a while after production before use. 

The first thing you might think of as a user is to shake the gallon for a well-mixed chemical. You are allowed to shake spray paint, not water-based polyurethane. Shaking is not allowed on woodworking or painting products. 

Stirring prevents bubbles, and shaking forms bubbles on top of the polyurethane. The moment you dip in your brush, it takes up the bubbles and leaves them on the surface you are painting, and they dry just like that. 

Leaving air on the brush

There are air pockets in your brush that cause bubbles when applying polyurethane. To avoid air on your polyurethane brush, dampens it with water under the sink. Soaking them will not affect your task. However, there is a need to remove dripping water using your hands before use. 

All the bristles have to be wet, and as they stick together, they glide well on the surface. When your polyurethane is oil-based, dampen your brush using a solvent-based product, but when it is water-based, water does the trick. 

If you choose to use solvent-based products on water polyurethane, you will deal with bubbles only, but more problems come with such a move. The finish will not be flawless. Please do not use a roller since it creates bubbles. 

Contaminated surfaces

Contaminated surfaces make polyurethane act funny as you are applying it on the surface. Sand down your projects to the extent that they look like bare wood or strip them down. That is the initial stage of the project.

Deep cleaning gets rid of dust and dirt. Use mineral spirits when cleaning. Mineral spirits are also solvent-based products, and they clean the surface well. As soon as you finish using the spirits, they evaporate instantly. Before you start using the polyurethane, make sure the spirits dry.

Use Shellac

Using Shellac first prevents bubbles on your surfaces. Make sure there is no contamination before applying the polyurethane. Shellac seals anything underneath, and that does not allow bubbles to show. Apply Shellac and make sure it dries first before polyurethane. Shellac stops your product from bleeding as well. 

How to apply polyurethane 


  • Sanding block
  • Shop vacuum


  • Lint-free cloth
  • Mineral spirits
  • Oil-based polyurethane
  • Sandpaper
  • Tack cloth
  • Wet/dry sandpaper
  • Automotive rubbing and polishing compound


  • Sand the surface
  • Remove dust
  • Seal the surface
  • Apply first coat
  • Remove bumps
  • Wet sand the first coat
  • Apply the second coat
  • For more coats, repeat the process

Bubbles in Fresh finish

Wiping the brush along the side of the can creates bubbles. It brings bubbles inside the mixture in the gallon. Take your project brush to the surface to start the process as it transfers the bubbles to the surface. 

You still have the chance to get rid of the bubbles whilst the polyurethane is still fresh. Dab the bubbles with the tip of the brush and run the project brush along the surface. By doing so, you are flattening the bubbles, and they disappear into the finish. 

Bubbles in a hard finish

There are chances that you will not notice the bubbles when the finish is still wet. That means the bubbles are hardened together with the rest of the finish. The chances of failing to notice bubbles happen to users who use the finished applicator aggressively. They continue to complete the project without monitoring the outcome. Another coat covers up the bubbles, but if it fails, you try other methods. 

How to get rid of dried polyurethane bubbles

Use steel wool

Start by using the 0000 as you try to rub the dry bubbles. Steel wool is graded in 0s and 0000 as the flexible and ultra-fine grade. It gives you a polish and cleans delicate surfaces (Source). Steel wool is so fine that it rubs bubbles with ease. 

There will not be a trace of scratches, and the surfaces become glossy. There is a need to buff out the whole surface to match if you are not giving the wood surface an additional coat. If the 0000 does not give you satisfactory results, try going down to 000 or 00 steel wool. Work on a smaller patch before you try out the whole surface. 

Use a sander

Sanding the surface can be the answer to rubbing off polyurethane bubbles. High grit sandpaper such as the 400 is the ideal tool for bubbles. It polishes off your work surface as it sand off the bubbles. 

Choose to sand the bubbled area to match the surface or sand the whole surface since that almost removes the affected area. Use an electric sander, and there are multiple models, such as a random orbital sander. 

Sanding is riskier than the mentioned methods. Be careful as you sand off the bubbles. You will lose the entire surface when you want to lose the bubbles only. 

A high grit sanding sponge ensures safety in removing the bubbles. Washable sponges bring less damage to the surface. Do not wet them but look for high-quality sanding sponges. Softer sponges perform better than hard ones. 

Starting all over 

If the bubbles are bad, starting all over again might be the best way to get rid of the bubbles. You may have tried the above methods, and they did not work, and the option might be to remove the entire layer and re-start the process. 

Starting all over again protects your surface more than sanding since the chances are high that you will damage your project surface. Poor quality sanders and hard ones damage the bubbled polyurethane surface. 

As you start again, working on affected areas saves time and reduces the chances of bringing more damage to the surface. Staining or painting the affected area can be the best option if you cover the bubbles. Starting again can be draining, but it is worth it. 

Thinning polyurethane 

When polyurethane is still inside the gallon, it is thick and hard. That makes it hard to apply, and the chances of using the product without making bubbles are high. Although manufacturers advise users to run with the one coat idea, there is a need for lore coats for durability and effectiveness. 

Thicker surfaces have multiple layers of protection, and they are a product of multiple, thinner coats. Just in case there are bubbles from the first coat, a second coat hides the bubbles. The question remains, how do you then the product? 

Mix the polyurethane mixture with solvents on a 3:1 ratio. Three parts represent the polyurethane, and one part, solvent. The solvents are mineral spirits used on both oil and water-based polyurethane. Thinning is a process that reduces the thickness of polyurethane.

David D. Hughes
Latest posts by David D. Hughes (see all)

Leave a Reply