There are many reasons why polyurethane can cause bubbles to appear on the surface of the wood. These include using the wrong brush, shaking the can of polyurethane instead of gently stirring it, and failing to prepare the brush before applying the polyurethane.
Can You Use A Torch To Get Bubbles Out Of Polyurethane?
There are two popular methods for getting rid of these bubbles: a torch and sandpaper. The torch method is only recommended for water-based polyurethane while the sanding method can be used on both water- and oil-based polyurethane.
When To Use A Torch To Get Rid Of Bubbles
The best time to use a torch on water-based polyurethane bubbles is immediately after applying the topcoat and before it dries. The blow torch method can be used between coats of water-based polyurethane to prevent bubbles from getting trapped.
Blow Torch Method
Many woodworkers use a small, handheld propane torch to get rid of unsightly bubbles in water-based polyurethane layers. The flame is passed over the bubbles until they melt. Passing over the bubbles too quickly will do nothing to get rid of the bubbles, but holding the flame in place for too long can cause burning.
Allow each coating to dry thoroughly, as recommended by the product, before putting on another layer.
When finished, the polyurethane should have a glassy finish that is completely free of bubbles.
Those who do not own a torch or who feel uncomfortable using one may find a hairdryer to be a suitable replacement. The trick is to use the hairdryer on the bubbles immediately after applying the polyurethane. The hairdryer will need to be set on its hottest setting to have any effect and the airflow should be set to low.
A heat gun can also be used to get rid of bubbles in water-based polyurethane.
Sanding Recommended For Oil-Based Polyurethane
The problem with using the torch method on oil-based polyurethane is that the polyurethane can catch on fire.
Instead, the sanding method is recommended for projects using oil-based polyurethane.
This method involves using 120 grit sandpaper to gently scuff out the bubbles after the layer of polyurethane has cured for three days. When finished, gently apply another layer of polyurethane.
How To Avoid Bubbles
Avoiding bubbles in the polyurethane is the best route to take.
Begin by gently stirring the polyurethane inside the can. Avoid shaking the can at all costs because this will cause small bubbles to form that will get onto your woodwork.
Clean the surface of your work. If you are using water-based polyurethane, wipe your surface off with denatured alcohol. For oil-based polyurethane, clean with mineral oils.
A good quality brush, such as a horsehair brush, should be used to apply oil-based polyurethane. A good quality nylon brush can be used with a water-based polyurethane. Many woodworkers recommend against using sponge brushes because they create more bubbles on the surface of your project.
Finally, stick to applying thin coats of polyurethane. Try to avoid applying heavy layers of polyurethane that trap dirt and bubbles. Thicker layers of polyurethane also take longer to dry between coats.