How to Apply Polyurethane Without Bubbles — Your Step-by-Step Guide

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It’s frustrating seeing bubbles on top of your polyurethane coats. Polyurethane turns into rigid film when it dries, and removing dried bubbles is tricky that might crack your finish. 

While it seems challenging to prevent, don’t get frustrated as there are effective ways to do it! In this guide, our wood experts will show you how to apply polyurethane without bubbles.

What Causes Polyurethane to Bubble?

Unlike spray paint, polyurethane is stirred before use. Polyurethane reacts when you shake the container, forming bubbles as it traps air inside. 

polyurethane products

The improper procedure causes bubbles to form, so make sure you stir poly before using it. Thick poly makes it harder for air to escape. Unsanded wood surface and unprepared brushes are also culprits.  

Exposing the Brush to Air

Air lingers on the bristles of a dry brush that create bubbles when applying polyurethane. So we suggest soaking your brush in thinner to avoid it. For oil-based, soak your brush in mineral spirits first or water if you’re using water-based polyurethane.

Also, it’s better to use finish brushes since bristles on roller brush catch more air. We suggest using natural bristle brushes on oil-based polyurethane and synthetic bristle brushes on water-based polyurethane finishes. 

Not Using Shellac

Shellac is commonly thin, and bubbles less likely to form on thin coats. Since shellac is a great sealant, it prevents air from entering. But if it does, shellac is thin enough, so bubbles usually pop out on their own. 

Zinsser shellac

If you top it with poly, let it dry first. They’re incompatible, and you would want to avoid cross-contamination.  

Shaking the Poly Container

Bubbles form when polyurethane wraps air molecules inside it. When you shake the can, air starts to mix with poly. Air pockets are then trapped, forming bubbles, which can ruin the finish’s smoothness.

Denser parts settle when poly is stored. But never shake before using it; stir it using a mixing stick.   

Contaminated Wood Surface

Polyurethane contains chemicals that behave badly when contaminated. Untarnished, unsanded, and dusty wood surface holds air molecules. When you put poly on top of it, air stays and forms bubbles. 

Poly turns into rigid films when it dries. Thus, bubbles remain permanent. It leaves scars if you pop it off, so always sand wood surface first. 

milled Cherry tree wood

After light sanding, you can wipe it with cloth drenched in mineral spirits to make sure you have clean and smooth surface before applying polyurethane. 

Applying Polyurethane Without Bubbles: Detailed Guide

Take a look at how to apply polyurethane without bubbles using a few methods:

Tools and Materials You’ll Need

Applicators: Brush Types vs. Rollers

Synthetic Bristle

Synthetic bristle brush is usually made of polymers like nylon. It is slightly non-porous but durable and does not suck much liquid. However, it’s a perfect brush for water-based polyurethane, but you can also use it on oil-based polys. 

paint brush
Natural Bristles

This brush type is made of real animal hair or fur. Depending on what animal is used, quality may vary. For example, bristles made of pig hair are usually durable and of quality.

Natural bristle paintbrush, like China bristle, is soft and smooth that works well on oil-borne polyurethane. However, a natural bristle brush isn’t recommended for rough wood grain or irregular surfaces.   

Foam Brush

Foam brushes can be used in paints, stains, sealers, and wood finishes. It is cheap, disposable, and easy to use. Since it’s a dense foam, it’s highly absorbent and can lead to thick coats. 

Foam Rollers

Foam roller goes well with water-based polyurethane. It is cheap and disposable. Although foam roller is ideal for wider projects, it can leave brush marks, absorbs much liquid, and poor application can cause bubbles. 

Natural Roller Covers

For oil-based, you can use natural roller covers to get the job done. This roller made of natural fiber hastens your job due to its wider coverage. 

KUPOO Home Decorator Foam Roller

However, roller is more susceptible to bubble formation. So better use high-quality roller and follow thorough application to avoid it. 

Synthetic Roller Covers

Like synthetic brush, synthetic roller cover is made of artificial polymer, good for rolling polyurethane that’s water-based.  

Preparing Your Brush

Soak your brush in thinning agent before you apply the polyurethane finish. For oil-based products, we recommend using China bristle or any natural brush. If you use water-based poly, finishing brush with synthetic bristle (especially made from polyester) works best.  

Step #1: Clear the Dust

Wipe wood surface with tack cloth to remove small particles. If you have unclean surface, dust will mix into polyurethane, resulting in large bubbles formation and rougher finish. 

vacuuming sanded floor

Vacuum cleaner, air compressor, and microfiber are good alternatives. Whatever you pick, be sure to clear off dust as much as you can. 

Step #2: Sand Away

This can be interchanged with step one. But we recommend sanding after cleaning the surface, especially on dusty wood. 

Sanding ensures smoothness and evenness of surface before you apply polyurethane. Remember to start from coarser to smoother grit, and follow the grain patterns when sanding. 

Start with 100 or 120 grit, then 220 grit, and so on. Only use finer grits, more than 440 if necessary. 

Step #3: Mix and Thin Polyurethane

There are two things to remember here. Oil-borne poly uses mineral spirits like thinner, while water-based polyurethane only needs clean water. Stir mixture with the amount of thinner needed and never shake. 

mixing polyurethane for air sprayer

Polyurethane boxes and thinning agents have instructions regarding the ratio. Take note of compatibility as well. Thinning polyurethane decreases chances of bubbles forming. Plus it reduces stickiness, making it easier to apply, smoothen, and clean the paint brush afterward. 

Step #4: Apply the First Poly Coating

This is crucial if you want to prevent bubbles in your finish. They say prevention is better than cure, and this applies to applying poly as well. Not only that it’s harder to remove pre-existing bubbles on dried finishes, but it’s also wiser to prevent bubbles from happening in the first place. 

Poly Brushing Techniques

Lay it Down

Allow excess solution to drip after you soak brush in polyurethane. To avoid forming bubbles, dip your brush deliberately. Don’t spatter it, press the brush on the side of container, or any sudden activity that might cause chemicals to react.

When stroking, the goal is to avoid entering of air. Start stroking from one edge. Go along with the surface when brushing, following straight lines or wood grains if applicable. Do it while applying as less pressure as you can.

Don’t brush the covered part again. Without cutting, gently slide brush onto the next row once you reach the other side of the edge. Overlap it for half an inch from the previous stroke to avoid marks of brush.

applying polyurethane over chalk paint

Avoid frequently dipping brush onto poly. If your mixture is right, one dip can cover more surface.

Overlaps have thicker surfaces. You can stretch it as you successively brush one stroke after another. That way, you’ll save more materials while efficiently avoiding brush marks, bubbles, and unevenness. Do this until you cover all surfaces. 

Feathering

Gentle strokes are key in feathering. This step doesn’t require dipping the brush again. All you have to do is caress the brush onto the first coat to smoothen things. 

Start from one edge to another. Unlike previous step — where you continuously stroke back and forth, forming a snake motion — you feather the coat in one direction this time. Overlap next stroke half an inch from the previous. Repeat until you make sure you feather out everything. 

Reapplication Time

More coats of polyurethane provide better protection. Apply multiple thin coats to reach desired thickness. 

Step #5: Sand Before the Second Coating

Some manufacturers claim their formulation no longer requires sanding after each coat, which our woodworkers proved to be true. But at the end of the day, sanding after each coat is still better than not sanding at all.

sanding table top before applying poly

Use sandpaper with grits more than 220, so your second coat can effectively stick to your first coat. Lightly sand like barely it’s touching the surface, following the brush stroke, and never use coarse sandpaper to avoid scratches.

While you wait for your coat to dry, clean previous brush by pressing out poly residues on all the bristles with paint thinner or mineral spirits before reusing it for the subsequent coats. 

If you let it dry overnight or over the course of a day, soak your brush in thinner for a few minutes. Let it dry after that so it can be reused the next day.

Step #6: Apply the Next Poly Coat

Remove all dust using cloth, microfibre, or clean rags before applying next coat. Then simply follow step #3, making it thicker this time. 

white rag

Step #7: Repeat the Process as Needed

The thicker finish, the better. Follow step #5 first. Sand before applying next coat. Then recoat by following step #4. But how many coats of poly do you need? Check this guide. 

Repeat this depending on what the circumstances are. Number of recoats is lesser if wood has basecoat. Ingredients are also factors. Water-based requires at least four recoats or more, while oil-based polyurethane requires 3-5 recoats.

How to Apply Oil-Based Poly Without Causing Bubbles

If you want to know specific steps for an oil-based polyurethane application, follow the steps below:

Step #1: Prep the Wood Surface

To apply polyurethane without bubbles using an oil-based product, ensure everything is set before commencing. Completely clear off dust on wood with any cloth (preferably tack cloth), vacuum cleaner, or dust blower. You can also wet sand the surface. 

Step #2: Pour some Poly on a Paint Tray

Stirred polyurethane settles quickly. It makes sense to pour it in a tray instead of a narrow-mouthed can during application. Putting some poly on a paint tray makes more seamless work, even working better on roller brush. 

Step #3: Apply Polyurethane

To avoid formation of air bubbles, use brush made of high-quality materials. Natural China bristle, for instance, is a quality brush for oil-based finishes. Brush surface with even and thin coating thoroughly.

using paint roller to apply polyurethane

Pop off bubbles with tips of bristles if you see one, and cover entire surface with poly in a single direction. 

Step #4: Let it Dry

Touch dry oil-based is enough for next coat, which only takes about 4-6 hours while water-based dries after couple hours. Even so, we’d let it dry for a day just to be sure it’s ready for the next coating. 

Step #5: Re-apply As Needed

When surface has base coating, recoat at least 2-3 times. Repeat the process 4-5 times if starting from bare wood. Fast-drying polyurethane claims it dries within 4 hours. Either way, re-apply coating only once a day for better results. 

Step #6: Let it Cure

When you’re done with final coat, let it sit first before using. Oil-based usually takes two days to cure. However, use it lightly and wait for it to totally cure for 2-4 weeks before fully using it. 

Also Read: Do You Need to Sand the Final Coat of Poly

How You Should Apply Water-Based Poly Without Bubbles

Bubbles on water-based poly can be avoided by being thorough. You can actually follow a  lot of pointers from the detailed guide above. 

Oil Base Polyurethane

Using high-quality synthetic brush, applying thin coats, and patiently drying and sanding between each coat reduce air bubbles to form.

How to Roll Poly Without Making Bubbles

Roller causes more polyurethane bubbles, so use high-quality roller wet with thinning agent if you really want to use one. Lambswool roller is ideal for oil-based polyurethane. Either pick microfiber or foam for water-based polyurethane. But is it really advisable to roll polyurethane on wood? Learn more here!

How to Avoid Bubbles While Applying Poly on Wood Floors?

Brush is too small for floors; better use T-bar or roller to make your work faster. T-bar, for example, is ideal for plank floors since it only follows a straight pattern. Meanwhile, roller is more flexible. If you have more complex flooring, use roller instead. 

As mentioned, ready your wood surface before you roll polyurethane to your floor. Clear dust, sand surface with fine-grit sandpaper, and dampen roller with thinner. Then, thin coat the surface. Repeat the process as many times as required.

How to Remove Bubbles from the Poly Coating

No matter how hard we try to avoid bubbles, they can suddenly appear and ruin your project. Don’t get discouraged. If you’re lucky and it’s not completely dry, you can pop off tiny bubbles through brushing; just apply a little pressure on larger bubbles. 

dresser top with brown polyurethane finish

Things become complicated when it dries and even harder when it fully cures. Some people use coarse sandpaper. We tried it, and it leaves scratches, so we suggest using fine-grit sandpaper, around 320. Only sand area with bubbles, so you’ll leave lesser scratches. 

However, larger bubbles may need coarser sandpaper with 100 grit or less. If it’s severe, use scraper or circular sander on the affected area and completely remove coating. Wipe the area with mineral spirits or any alcohol and apply new coating.

Bubbles on Fresh Poly Finish

If poly is still fresh, then it’s far from over. Simply stroke the brush tips on the bubbles, and it will pop off. But if it doesn’t, dab it with little pressure or dampen brush with a bit thinner. It might be harder on larger bubbles, so be thorough. Follow original stroke so you won’t ruin your finish. 

Bubble on a Hard Poly Finish

This annoys us, but sometimes we overlook. And before you know it, bubbles already hardened along with the finish, and it’s too late to stroke it with brush. 

bubbles on polyurethane finish

As long as you’re particular and careful, it’s avoidable. This should be avoided at all costs because you’d have to completely remove the affected area to get rid of it.

How to Get Rid of Bubble From Dried Poly

Sander Method

The sander method is your go-to to fix polyurethane bubbles. Use fine-grit sandpaper on minor and smaller bubbles, and only use coarser grit if necessary. Using electric sander also works if bubbles are hard enough for sandpapers. 

The downside is sanding can be messy. Poly may harden when it dries, and sanding can leave scratches and even crack unaffected parts– doing more damage than good.

If this is the case, choose high-grit sanding sponge. It’s smoother and softer than regular sandpaper; hence, it’s unlikely to cause damage.

Steel Wool Method

Steel wool is a gentler option. Starting with super fine steel wool (grade 0000) [1] decreases error chances. Also, steel wool further polishes the finish. 

person holding steel wool

If finer wool can’t remove bubbles, go up to coarser woo 0000-0. Note that grades past 0 and 1 can be rough and might leave minor marks. 

Although steel wool requires more time and effort than sanding, it can get rid of bubbles with lesser shortcomings than sander.

Start the Process Again

This can be time-consuming, but this is the best option. Only decide refinishing as last resort since it requires lots of effort, time, energy, and even money. 

Restarting the process gives you another chance to apply polyurethane better. 

Thin Polyurethane

Thick polyurethane has better protection against outside elements, and it goes both ways. Once air gets stuck inside the mixture, it’s harder for air to escape on thicker barriers. And it’s harder for you to get rid of bubbles when it dries. 

applying polyurethane

It’s obvious that thicker finish is better. The trick here is to have multiple recoats of thin polyurethane. Soaking your brush with thinner also helps.

FAQ

Is it okay to use a sponge brush to apply poly?

You can use a sponge when brushing poly. It’s effective on smaller furniture and can smoothen your finish. However, the sponge is inefficient on larger projects. A sponge may leave brush lines and creates bubbles. Using foam brush is more effective, and only use a sponge if it’s the only thing available. 

Can I use a hair dryer to remove the bubbles?

The heat and pressure of hair dryer will pop out the bubbles and smoothen the surface of your finish. The tricky part is — hair dryer releases air. If used poorly, it can lead to more bubbles. It’s wiser to follow proper procedures rather than letting bubbles form and getting rid of them later. 

Conclusion

Buying quality brush and top-of-the-line polyurethane won’t cut it. You must know the proper ways how to apply polyurethane without bubbles. 

Be particular with the proper brushing and coating techniques, and know what thinning agent works best. Just be thorough and patient, and never shake your container.

Robert Johnson is a woodworker who takes joy in sharing his passion for creating to the rest of the world. His brainchild, Sawinery, allowed him to do so as well as connect with other craftsmen. He has since built an enviable workshop for himself and an equally impressive online accomplishment: an extensive resource site serving old timers and novices alike.
Robert Johnson
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