How to Apply Polyurethane Without Bubbles? (& Fix Them)

Sometimes, after applying polyurethane, the finish develops bubbles. So, why does this happen, and how to avoid it?

To apply polyurethane without bubbles, thin it before applying. Also, avoid using a roller or foam brush to apply it. Instead, use a bristled paintbrush or spray gun to apply polyurethane.

Use water to thin water-based polyurethane and mineral spirits to thin oil-based polyurethane. To avoid over-thinning it, you must thin it in a ratio of 1:3 (or 1:4).

So, the bubbles will form if you apply thick coats, use a bad roller, or if you shake the container before applying it. 

Why Does Polyurethane Develop Bubbles?

A polyurethane finish develops bubbles if you use a bad paint roller to apply it. This is because paint rollers trap air pockets in the coating as you roll the paint (sealant) over the surface. These air pockets become evident in the coating, and as the sealant dries, the air pockets will form bubbles in the coating.

The polyurethane finish develops bubbles if you apply it over a porous surface (such as fresh wood). This is because porous surfaces have small air pockets trapped in the wood grain. So, once you apply it over porous surfaces, the air (stuck in the wood grain) will lift the coating and cause bubbles in the finish.

If you stir the polyurethane too much or shake the container too hard before applying, the finish will develop bubbles. Also, if you use the wrong brush, the finish may develop them. You must use synthetic bristled brushes to apply water-based poly, and natural bristled brushes to apply oil-based poly. If you use the wrong brush, the finish may develop bumps.

You may also notice them forming if you paint over a wet or moist surface, or if you don’t leave enough dry time between coats. So, bubbles will only form if you don’t do proper prep work or if you use the wrong painting equipment. 

Tiny Bubbles Will Disappear

Tiny bubbles will disappear as the finish dries they are caused by trapped air pockets, and once the paint starts to harden, they will disappear. The drying process of polyurethane will cause the tiny bubbles to pop and disappear. On average, they will go away within 10 minutes. 

However, large bubbles won’t go away because they contain a mixture of paint and air. So, the bubbles will dry into the finish and cause a bump.

To remove the large paint bubbles, scrape them off and sand the polyurethane coating to even out the finish. After sanding, you must touch up the affected areas.

How To Apply Polyurethane Without Bubbles?

Applying polyurethane without bubbles is easy; here are the tools you need:

  • Medium and fine-grit sandpaper
  • Wire brush
  • A portable vacuum
  • A spray gun or a bristled paintbrush
  • Paint thinner (mineral spirits or water)
  • Polyurethane

1. Prep The Material To Be Painted

Prep The Material To Be Painted

First, prep the surface you want to paint. Polyurethane can develop bubbles if the surface is wet, has holes (or crack), or is dirty. If the surface is wet, wait until it dries before sealing it.

If the surface has holes or cracks, there will be air pockets inside those holes. These air pockets will cause bubbles in the finish, so you must repair the cracks and holes to avoid them. For wooden surfaces, seal the crack with wood filler. For concrete surfaces, use concrete paste. 

Also, you must clean and scrub the surface to get rid of dust and dirt that may cause bubbles. To clean, use a brush and a portable vacuum. 

2. Sand The Surface

Sand The Surface

After prepping the surface/material, you must sand it. First, start sanding with medium-grit sandpaper (180-grit) to remove bumps and imperfections from the surface. Then, use fine-grit sandpaper to smoothen the surface.

If the surface is painted and you want to apply polyurethane over it, sand it with ultrafine-grit sandpaper. Avoid using medium-grit or coarse-grit sandpaper over painted surfaces, as it can remove the paint.

After sanding, vacuum the dust on the surface.

3. Thin The Polyurethane

Thin The Polyurethane

Polyurethane is more likely to develop paint bubbles if the coating is too thick. So, to prevent them, thin the sealant.

If you use a sprayer, thin polyurethane in a ratio of 4:1 (4 parts poly, 1 part paint thinner). For a brush or roller, thin it in a ratio of 3:1. To thin oil-based polyurethane, use mineral spirits. To thin water-based polyurethane, use water as a paint thinner.  

To thin polyurethane:

  1. Pour it into a large plastic container (measure how much you poured).
  2. Add paint thinner (mineral spirits or water, depending on the type of polyurethane).
  3. Use a turning stick or paint mixer to mix it and the paint thinner. 
  4. Don’t stir too hard, as it can cause bubbles. 
  5. After stirring for 5-10 minutes, check the flow of polyurethane.
  6. If it has a consistent flow and color, you can use it. If it doesn’t, stir again. 

5. Apply The Polyurethane

Apply The Polyurethane

To apply polyurethane:

  1. Use a paintbrush or sprayer. 
  2. Apply 3 coats of polyurethane for proper coverage. 
  3. Wait until one coat dries before applying the next one. 
  4. Sand between coats (except for the final coat) with 320-grit sandpaper. 
  5. Allow the final coat to dry fully (cure) before using the surface.

Rolling vs Spraying Poly 

You should spray polyurethane to avoid bubbles in the finish. To spray it, you must thin it. Since the sealant is thinned, it has fewer chances of developing them. Also, the sprayer applies it at high speed, meaning the finish sticks to the surface better and prevents bubbles.

You can purchase spray-on polyurethane paints that come in aerosol spray cans or load the polyurethane into a spray gun. If you use a spray gun, thin it before spraying it (ratio of 4:1).

Rolling polyurethane can cause bubbles because the roller can trap air pockets in the coating and lay thicker coats. This means the finish will develop bubbles because of the air trapped in the roller. To avoid them while rolling poly, thin it or apply it slower. 

Fixing or Removing Bubbles

To fix dried bubbles in polyurethane, sand them off with fine-grit sandpaper (320-grit). Sanding will remove the them and leave behind a smooth coating. Here’s how to do it:

  1. Wipe the finish with a clean rag to remove dust nibs that can damage the sandpaper.
  2. Use 320-grit sandpaper to wipe the finish.
  3. Vacuum dust from the finish.
  4. Spray a coat of polyurethane to touch up affected areas.
  5. Polish the finish with linseed oil.

Final Words

You can prevent bubbles in a polyurethane finish by properly prepping the surface. You must allow each coat to dry before applying the new one. Also, using a paintbrush or sprayer instead of a roller minimizes the chances of a bubbled finish.

If you notice them in a dried finish, you can remove them by sanding the coating. 

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