Most times, DIYers notice bubbles forming during and after applying polyurethane. So what causes the bubbles and how can you apply polyurethane without bubbles? Here is the answer to that.
Bubbles usually form in polyurethane coating when you apply thick coats or use a bad roller to apply the polyurethane. Bubbles also form in polyurethane coating when you shake the polyurethane can too hard before applying the paint.
To apply polyurethane without bubbles, you need to thin the polyurethane down so you can apply lighter coats. You should thin oil-based polyurethane with mineral spirits and water-based polyurethane with water.
Also, avoid using a roller or foam brush to apply polyurethane. It’s better to use a bristled paintbrush or a spray gun to apply the polyurethane to prevent bubbles in the paint.
But that’s not all. Let’s dive in to find out why polyurethane paint gets bubbles.
Why Does Polyurethane Get Bubbles?
Polyurethane paint gets bubbles for different reasons. The first reason is when you apply the paint with a roller. Paint rollers trap air pockets in the polyurethane coating as you roll the paint over the surface. These air pockets become evident in the paint. As the polyurethane starts to dry, the air pockets form bubbles in the paint coating.
Polyurethane paint also develops bubbles when you apply the paint over a porous surface like fresh wood. Porous materials like wood usually have tiny air pockets trapped in the wood grain. If you apply polyurethane over such porous surfaces, the air in the material will lift the polyurethane coating causing bubbles to form in the paint as it dries.
You can also notice bubbles in the polyurethane paint when you apply expired polyurethane. If the paint has gone bad, it might develop bubbles.
Bubbles also appear in polyurethane while stirring or shaking the paint. Usually, you’ll have to stir the polyurethane before applying it. If you stir too hard, bubbles will form in the paint. Also, if you shake the paint can too hard or too quick, bubbles will form in the paint.
Bubbles also form in polyurethane when you use the wrong brush. You are to use synthetic bristled brushes for water-based polyurethane and natural bristled for oil-based polyurethane. The wrong brush will give the wrong application and can cause bubbles.
You may also notice bubbles forming if the surface that was painted is wet. If you apply polyurethane over a moist area, the paint may develop bubbles. Also, if you don’t leave enough dry time between coats, the polyurethane paint might have bubbles.
The bottom line is the polyurethane paint will only develop bubbles if you don’t do proper prep work or if you use the wrong painting equipment.
Will Tiny Bubbles In Polyurethane Go Away?
The tiny bubbles in your polyurethane will disappear when the paint dries fully. As explained earlier, these bubbles are usually trapped air pockets in the paint. As the polyurethane begins to dry, the paint coating will begin to harden.
This drying process of the polyurethane paint will cause the tiny bubbles in the paint to pop and disappear. Usually, these tiny bubbles will go away in less than 10 minutes after they appeared.
However, you should know that larger paint bubbles will not go away. Large paint bubbles contain a mixture of paint and air. The large bubbles will dry into the paint coating and result in bumps in the finish.
To remove the large paint bubbles, you’ll have to pop them with a needle or scrape them off. After, you’ll need to sand down the polyurethane finish and touch up the affected parts.
Should I Roll Or Spray To Avoid Polyurethane Bubbles?
You should spray polyurethane to avoid bubbles in the paint coating. When you spray the paint, the paint will have a thin flow. This makes it less likely for it to develop bubbles. Also, spraying the paint helps to achieve a smooth finish and proper coverage.
If you roll the polyurethane, it will develop paint bubbles because the roller will trap air pockets in the paint and lay thicker coats. This is a recipe for a bubble-infested polyurethane paint coating.
You can purchase spray-on polyurethane paints that come in aerosol spray cans or you can load the polyurethane into a spray gun. If you’ll be using a spray gun, ensure to thin the polyurethane before spraying it.
Let’s check out how to apply polyurethane without bubbles.
How To Apply Polyurethane Without Bubbles?
To apply polyurethane without bubbles, you’ll need the following tools and supplies:
- Medium and fine-grit sandpaper
- Wire brush
- A portable vacuum
- A spray gun or a bristled paintbrush
- Paint thinner (use mineral spirits for oil-based polyurethane and water for water-based polyurethane)
- Polyurethane paint
Here is a guide on how to apply polyurethane without bubbles
1. Prep The Material To Be Painted
The first step is to properly prepare the surface you want to paint. As explained earlier, polyurethane can develop bubbles if the surface is wet or riddled with pores. If the surface is wet, you need to let it dry before you can paint over it with polyurethane.
If the surface has cracks or holes, there will be air pockets inside those holes. The air pockets will cause bubbles in the polyurethane so you need to repair cracks and holes. If it’s a wooden surface, seal the cracks with wood filler. If it’s a concrete surface, use concrete paste.
You’ll also need to clean and scrub the surface to get rid of dirt and grime that may cause bubbles in the polyurethane. Use a brush and a portable vacuum to remove dust and dirt. Next, you should…
2. Sand The Surface
After repairing and cleaning the surface, ensure to sand it. Start with medium grit sandpaper like 180-grit to remove bumps and pimples on the surface.
Then work your way up to fine-grit sandpaper. This will help to smoothen the surface so the finish can come out smooth. After sanding, vacuum the dust on the surface.
3. Thin The Polyurethane
The next step is to thin the polyurethane. As explained earlier, polyurethane paint is more likely to develop paint bubbles if you apply thick coats of the paint. To thin the polyurethane, you’ll need to pour the paint into a large plastic container and add a paint thinner.
For water-based polyurethane, use water to thin the paint and use mineral spirits to thin oil-based polyurethane. You’ll need 1 part paint thinner for 2 parts of polyurethane paint if you are to spray the polyurethane. If you want to apply the polyurethane using a paintbrush, you can go with 1 part paint thinner to 3 or 4 parts of polyurethane paint.
After adding paint thinner, you should stir the paint for a few seconds. Ensure not to stir too hard or you’ll cause bubbles in the paint.
4. Apply Paint Primer
Before you apply the polyurethane, ensure to apply at least a coat of stain-blocking primer on the surface.
The primer coating will help to seal the surface to prevent any imperfection from damaging the polyurethane finish. Use a latex primer for water-based polyurethane and an enamel primer for oil-based polyurethane.
5. Apply The Polyurethane
The final step is to apply the polyurethane paint. You’ll need 3 coats of polyurethane for proper coverage on the surface. It’s better to spray the polyurethane but you can also brush the paint if you are experienced.
Don’t use a roller to apply polyurethane as this might cause paint bubbles. If you’ll be using a paintbrush, ensure to put the tip of the brush in the paint thinner before dipping it in the polyurethane. This also helps to prevent bubbles from forming in the paint.
Also, ensure that each coat of polyurethane is dry before a re-coat. You should also sand each polyurethane coat lightly before applying the next coat. Use ultra-fine grit sandpaper like 320 grit to sand each coat of polyurethane before the next coat.
You don’t have to sand the final coat of polyurethane. When all the coats are dry, leave the paint to cure for at least 3 days before light use. Let the paint cure for 5 days before you move furniture on it.
So now you know how to apply polyurethane without bubbles but how do you fix bubbles in dried polyurethane? Let’s check that out.
How To Fix Bubbles In Dried Polyurethane?
To fix bubbles in the dried polyurethane, you need to sand the bubbles off. To do this, you’ll need very fine sandpaper or a sanding block. 320-grit will work fine to remove the bubbles. Here is a guide on how to fix bubbles in dried polyurethane:
- Wipe the polyurethane finish with a clean rag to remove dust nibs that can damage the sandpaper
- Use 320-grit sandpaper to wipe the polyurethane paint
- Vacuum dust from the polyurethane finish
- Spray a coat of polyurethane to touch up affected areas
- Polish the polyurethane finish with linseed oil.
You should know that you can only remove tiny bubbles in polyurethane. If the polyurethane has developed large bubbles, it would be very difficult to remove it. To fix large bubbles in polyurethane, you’ll have to remove the entire finish.
Overall, you can prevent bubbles in your polyurethane paint by carefully prepping the surface before applying the polyurethane. You should also use the right paintbrush and remember not to use a roller. Also, allow each coat of polyurethane to dry before re-coating. This also helps to prevent paint bubbles in the polyurethane.
If you notice bubbles in the polyurethane when it has dried, you can remove it by sanding the polyurethane finish. You can’t fix large bubbles in the polyurethane paint by sanding. You’ll need to strip off the entire coat and re-apply the polyurethane.