It’s not an uncommon wood finishing problem, but it can be the most annoying one to have to deal with.
From polyurethane to shellac to oil finishes, all of them can sometimes fall foul of air bubbles marring their smooth final coat.
Now usually, as long as varnish dries slowly, air bubbles should pop all on their own.
But, what can you do if they don’t?
Well, in this post you’ll learn what the difference is between dry varnish and cured varnish — and how it relates to bubbling varnish. You’ll also find out what it means to ‘tip-off’ a wood finish — and how it can help you get that perfect final smooth finish.
And keep reading to discover what you can do to fix air bubbles in already dried varnish.
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So What’s Causing All Of Those Bubbles In Varnish?
Short answer? Chemical Reactions.
You see, when a wood finish cures, it undergoes a chemical reaction. This is opposed to when they dry, (a completely separate process to curing), which is a case of evaporation.
Now, drying (evaporation) turns varnish from a liquid into a film. Yet it is the curing process (chemical reaction) that turns varnish into a hard resinous substance.
But, here’s the thing; chemical reactions generate a fair amount of heat. So much so, that the heat will warm up anything else nearby.
So Chemical Reactions Cause Heat. Tell Me, Why Does This Matter Again?
Well, the heat generated will evaporate some of the moisture content naturally stored inside wood fibers. And when moisture evaporates — but is trapped in the still-curing liquid beneath a dry wood finish film — it can form air bubbles.
Generally, this is not a problem in and of itself. Air bubbles will rise, pop — and ultimately escape from a wood finish — all on their own.
However, in order for this to happen, we need wood finishes to dry fairly slowly. If varnish dries too quickly, those rising bubbles will not have time to escape.
This is why we sometimes add slow-speed thinners to certain wood finishes, (such as polyurethane), to help slow down drying.
Is Trapped Air (Inside Wood) The Only Reason Why It’s Bubbling?
That tends to be the main reason, but not always.
Another reason why air bubbles form can be due to a lack of brush preparation.
You see, you need to prep a brush before you dip it into any can of wood finish. So, with a solvent-based finish such as varnish, you need to dip your brush in a solvent (such as Mineral Spirits) before you then put that brush into the can.
The solvent will wet the brush, and remove any potential air pockets nestled in those bristles.
And yet another reason for air bubbles can come from shaking the container the varnish is stored in.
This possible cause is less of an issue with very thick oil-based finishes. Yet it can still sometimes create little air bubbles inside liquid varnish.
OK. So How Do I Get A Smooth Varnish Finish (With No Added Bubbles)?
Ideally, you should follow a few best practices when it comes to applying varnish onto wood:
1). Never Shake The Can
If you need to blend it, then stir varnish around inside the container. Never shake it.
2). Only Apply Over Cured Finishes
If you’re using varnish as a top coating sealant, then make sure the finish underneath has dried and cured first.
This is an important step, as slow-drying finishes (such as Linseed oil or Tung oil), can take weeks to cure.
3). Prepare The Brush Before You Dip It Into The Container
Dip the brush in solvent, such as mineral spirits, (and squeeze off any excess), before you dip it into oil-based varnish.
And when you apply, always stick to applying very thin coats of varnish.
Related Post: Can You Apply A Coat Of Varnish Over Tung Oil?
But I’ve Already Applied It! How Do You Get Bubbles Out Of Varnish While It’s Still Wet?
You’ll need to ‘tip off’ that varnish to get rid of those bubbles. And tipping off involves using a bristle brush to press — and pop — those bubbles.
This wood finishing technique involves lightly swiping the bristles of your brush along the wood grain.
Now, tipping-off tends to be a bit hit-and-miss when it comes to getting rid of bubbles in a wet finish. Still, if you want to get it right, there are two key things you need to do:
1). Use A Clean Brush
The brush should not have any varnish on it, instead keep that brush unloaded.
Also, dip the brush in mineral spirits or thinner — and squeeze off any excess — before you start.
2) Keep The Pressure Light
Try to keep the pressure very light as you move the brush.
You do not want the bristles of the brush to bend as you move it, otherwise you’re applying too much pressure.
But What If The Varnish Has Already Dried? How Do You Get Bubbles Out Of Dried Varnish?
At this point, your last-gasp option is to sand away the sections that contain those bubbles, and then reapply:
1). First off, wait for the varnish to dry and harden (you may need to wait for it to wholly cure too).
2). Afterward, grab some 80-grit sandpaper and begin sanding away those troublesome bubbles.
3). Once those bubbles are gone, you can now use 100-grit to 120-grit sandpaper to even out the surface once again.
4). Last, but not least, you can go ahead and apply a fresh coat of varnish.
To Wrap Up, Here Are The 3 Key Takeaways From This Post…
- 1). Air bubbles are the result of varnish drying a bit too quickly.
- 2). If varnish dries too fast, the pockets of air naturally found inside wood grain won’t have time to escape. Instead, they will become trapped underneath the varnish.
- 3). You can remove bubbles from wet varnish by ‘tipping off’ those bubbles. You do this by lightly brushing the tip of a bristle brush along the surface.