How to Apply Polyurethane Without Bubbles

Bubble-free polyurethane application technique
How to apply polyurethane without bubbles Image

Very few finishes can make furniture look sexy like polyurethane, but the danger of beauty is that it can easily be ruined. In this case, by bubbles.

Learning how to apply polyurethane without bubbles is one of the most challenging skills to master. Kidding! It literally takes just a few minutes (hours), a bit of patience (a lot), and this article (+ some videos).

Tools you Need to Apply Polyurethane

To apply polyurethane to polished or stained wood, you would need:

  • 180 grit sanding block
  • 220 grit sanding block
  • Superfine grit block
  • You may also use 120 grit sanding block for a new project, or 80 grit for an old one
  • Vacuum cleaner
  • Tack cloth
  • Bristle paintbrush (read the best brush for polyurethane review to choose one)
    • Use a natural bristle paintbrush for oil-based polyurethane; or
    • Use a synthetic nylon bristle paintbrush for water-based polyurethane application
  • Thinning agent
    • Use mineral spirits for oil-based polyurethane application; or
    • Water for water-based polyurethane
  • Polyurethane – it could be satin, semi-gloss, or gloss

Other items you would need are a storage container, a cleaning container, and a drop cloth. You may also need a sponge and lint-free rags. Ideally, you should have a workstation in a well-ventilated room for your health and safety, but that’s not technically a tool.

How to Apply Polyurethane Without Bubbles in 7 Steps

Before you begin, you may want to put a drop cloth underneath the project to prevent stains on your floor or workbench. This is an optional first step, so I have listed it here separately.

Now, let us begin.

Step 1: Get Rid of Dust

The key to starting off right is to get rid of dust – not just some of it but all of it.

There are two ways to get this done. The first is with a vacuum cleaner. Slowly hover every inch of the piece of wood you are working on until all the dust is gone.

You can also remove dust with the help of an air compressor and a tack cloth. Note that you will also need to get rid of dust after step two.

Step 2: Sandpaper

This is where the real fun begins. While it can be okay to start sandpapering before you remove dust, you definitely should not try to apply polyurethane without sandpapering.

The job of the sandpaper is to ensure that the piece of furniture is actually smooth, despite whatever the person at the DIY store said.

Sandpaper is measured in grit, which is how coarse the sheet is. The toughest and the first one you should use on old furniture is 80 grit. This will handle all the stubborn rough spots on your project until it is level.

Once its level, introduce 120 grit sandpaper and move on to 180 grit or 220 grit. 220 grit is the finest, or least coarse, leaving your woodwork smooth and splinter-free.

Step 3: Get Rid of Dust, Again

When you are done sandpapering, you need to give it a proper wipe down. This serves a dual purpose: the first is to obviously get rid of dust, but the second is to ensure that the wood is indeed smooth.

To get rid of the dust this time, use a tack cloth. The cloth should glide as you wipe down. If it gets snagged along the way, then you know your sandpapering isn’t done.

Step 4: Prepare the Polyurethane

After spending a long time on shelves, polyurethane tends to separate in the can. So the first thing you need to do is mix it, but DON’T shake it. Shaking adds bubbles, making your job a lot harder.

Use a mixing stick or any stick to slowly stir it. Then, depending on how thick the polyurethane looks, you can either apply it as it is or thin it. To get the best finish, you need at least two coats, so thinning will prevent you from applying too thick a layer.

To thin the polyurethane, dilute 3 parts of it with 1-part solvent. Pour the poly and the solvent into a separate container and gently mix them.

Step 5: Apply the First Coat

Dip the brush (natural bristle for oil-based poly and synthetic nylon for water-based) into the polyurethane mix gently, and allow the excess fluid to drip back into the bowl. Do not splash the brush or wipe off the excess on the side of the container; otherwise, you will introduce bubbles.

Start from one edge of the piece of furniture and gently stroke along the grain. You don’t need to apply pressure on the brush and only use a bit more than the tip, roughly ½ an inch.

Work the brush back and forth on each ‘row’ until you have coated the entire surface. Then, do the same with the sides and edges.

Once you are done, let the polyurethane dry. This may take up to 24 hours.

Some manufacturers suggest that you may apply the next coat when the first one is still wet because it adheres better, but this may not give you the chance to get rid of the bubbles. However, I tend to err on the side of caution.

Step 6: Sand Before the Next Coat

Do you have to sand between coats of polyurethane? Yes, after the first coat has dried, use 220-grit sandpaper along the grain of the wood. Sanding will help get rid of any bubbles and smooth over any high spots.

You won’t need to press too hard as you are not trying to get rid of the first coat. Check to see that the bubbles are all gone, then use a tack cloth to get rid of the dust.

Step 7: Apply the Next Coat

For the second coat, you are going to apply the polyurethane a bit thicker than the first. So, again, apply with little pressure along the grain in a sweeping motion back and forth. Some people recommend you only brush in one direction, but it’s unnecessarily tedious.

The wood absorbs the finish and glistens whether you stroke in one direction or two, as long as it follows the grain.

Repeat steps 6 and 7 until you achieve the desired look.

How To Apply Water Based Polyurethane Without Bubbles

To apply water-based polyurethane without bubbles, it’s a good idea to apply multiple thin coats. Use a quality synthetic polyurethane brush instead of a natural brush. And before applying, dampen the polyurethane brush with water to avoid bubbling, puddling, or running. Stir the poly and remember never to shake the can because it might create bubbles.

Are You Ready?

Now that you know how to apply polyurethane without bubbles, are you ready to begin your next project? Personally, I would love to see pictures or videos of how it turns out, as I hope this article was simple enough to follow.

Before then, let me know if you have any questions in the comment section, and do share this with any of your woodworking pals.

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