How to Get Bubbles Out of Dried Polyurethane

image of a polyurethane finish without bubbles

All painters have, at some point, experienced a bubble problem in their dried polyurethane. Over the years, theories rose from this problem— about the causes of bubbles.

Here’s how to get bubbles out of dried polyurethane.

Well, getting bubbles out of a dried polyurethane involves steps such as cleaning of the surface, stirring the formula to ensure evenness, applying thin layers, and sanding between the coats.

So, if you have a bubble incident or are just looking for ways to avoid them, read on for more information.

Main Causes Of Bubbles In Polyurethane and How To Avoid Them

Using the wrong brush
For an excellent finish, you need to choose the right kind of brush based on the type of polyurethane you intend to use.
Use a brush with natural bristles (made from animal hair) for oil-based polyurethane. They carry a lot of your finish from the can to your surface and last longer. Water-based polyurethane, on the other hand, works better with synthetic/nylon bristle brushes.

If you use natural bristle brushes with water-based polyurethane, the brush’s bristles clump together and lose their shape, and they are more likely to leave brush marks behind.

Improper brush preparation
Before applying polyurethane onto your prepared surface, dampen your brush in a solvent to remove air bubbles caught between the bristles and in the ferrule.
Soak your brush in water(for water-based polyurethane) or mineral oils (for oil-based polyurethane). Soaking the brush also softens the bristles to make your application even better.

Let the brush soak until air bubbles stop coming out from the ferrule, then remove the brush and gently wring out the solvent. You can also drag the brush across an old newspaper (gently) to dry it out, then use your brush as you usually would.

Shaking the cans before application

The two most common (proven) beliefs among wood finishing painters are that you should never shake the can or wipe your brush along the side of the can to get rid of excess product. Both activities lead to bubbles forming in the poly, and you may transfer them to the surface as you apply the finish.

How to Apply polyurethane and avoid Bubbles in the Dried Polyurethane

Clean your work surfacePolyurethane accentuates all the features of your surface – including the inconsistencies. So, before applying the first coat of your finish, take some time to properly sand the area then get rid of the sanding debris using a vacuum and a tack cloth. You can also use a cloth dipped in mineral oils(For oil-based polyurethane) or denatured alcohol( for water-based polyurethane) to wipe the surface.

Stir the contents of the can – Gently stirring the polyurethane before application helps get rid of gas bubbles that had formed before. Doing this reduces the risk of you transferring those bubbles to your surface and ruining your finish.

Apply thin coats, gently – A polyurethane finish looks better when the finishing surface has thin coats. Thick layers tend to be uneven and are more likely to attract dust. They also take longer to dry between coats.

Tip-Off” each section of the surface – Tipping off refers to running your brush across the surface immediately after applying your finish. Do this after each new application to minimize brush marks and bubbles.

Sand between coats – After applying your first coat, let the poly dry entirely before applying the next layer. Sand the surface to get rid of dust or debris that got stuck there during the drying process. Then apply the next coat.

NOTE: Before sanding, test on an inconspicuous part of the surface to ensure that the poly is completely dry. If the finish “balls up” or seems wet, let it dry some more.

Getting Rid of Bubbles In Your Dried Polyurethane Finish

1.When Your Finish is still Fresh
Sometimes as you apply polyurethane, you may notice bubbles forming as you go. To get rid of these before your varnish dries up, dab on the bubbles with the tip of your brush or brush very lightly along the surface you are finishing to flatten the bubbles.

2. After Your Finish Dries
If you miss the bubbles when the polyurethane is still fresh, you can even get rid of them after the finish has dried. Do this by sanding out the bubbles then apply a fresh coat of the product.

How to Sand Out Bubbles from your finish

Step One – Use 120 grit or finer sandpaper to scuff sand the bubbles. If the bubbling is extensive, you may use a palm sander. The trick here is to sand very lightly.

Step Two – Use a damp cloth to wipe away the sanding dust.

Step Three – Apply a fresh coat of polyurethane, moving the brush slowly to avoid creating more bubbles.

Moving of the brush so quick causes trapping of air between the bristles creating air bubbles that then get transferred to the surface.

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