If your wooden surface requires a clear and durable finish, a polyurethane coating is essential.
It offers long-lasting results on many surfaces such as tables, flooring, countertops, etc. Since polyurethane is resistant to water, chemicals, solvents, impact, abrasions, etc., it’s a great option to protect your surface from damage.
Apart from providing physical appeal, this type of finish also strengthens your surface. However, most people struggle with deciding the number of coats to add and how to add them. Most people also hesitate to research more before getting into the process. Therefore, this topic will provide you a detailed insight into it and answer every possible query or confusion you may have.
What Is Polyurethane?
It is either an oil or water-based substance. In an oil-based polyurethane varnish, alkyd is a very common substance. It is sometimes combined with a polyurethane resin to give your product better strength due to its heat and water-resistant properties.
Pure polyurethanes do not have any alkyd resin and are often two-part products. They can cure by adding moisture and heat etc. This product is very popular in the furniture industry as its solid content is high, and the performance is also up to par. Having good solid content means there isn’t enough solvent to get lost in the atmosphere.
In water-based products, polyurethane is often combined with an acrylic resin, but they are weaker than oil-based ones. In case you would like to explore an alternative, there are also products with both oil and water-based properties. Since there are fewer chemicals involved in it, it is safer and also dries quickly. The water-based properties allow it to be cleaned easily, while the oil-based part ensures better durability.
The amount of polyurethane coating your surface needs depends on your project’s requirements and what type of product you’re using. The pros and cons of different polyurethane types differ; therefore, the choice should only rely on your project’s requirements.
How Many Coats Can You Apply With a Water-Based Polyurethane?
Your wood gets more clarity and better highlight when your water-based product is dry. When wet, the color it has gets milky. One of the best features of any water-based item is that it doesn’t create a mess and is very easy to clean. Moreover, the finishes they offer don’t get yellow easily. Apart from being ecological, it is also safe to use and the most viable option for an amateur.
This type of polyurethane dries faster and also levels itself well. No matter how much polyurethane coating you put, a water-based product will make the process quicker. However, like any other product, this one also has its downsides, such as:
- The final result is not as durable as you would expect it to be
- There may even be a lift in the wood grain
Therefore, you may need to apply several layers of this type of varnish for a smooth surface. Since its consistency is liquidy, you may need to apply more coats than you would with an oil-based varnish. On average, about five coats should be good enough. Since it doesn’t take a lot of time to dry, you won’t have to worry about the overall time, no matter how many layers you apply.
How Many Coats Can You Apply With an Oil-Based Product?
As it is already established by now, oil-based finishes are way stronger than water-based finishes. However, since they are also thicker, it can impact the amount of polyurethane coating that should be sufficient for your surface. When the finish is completely dry, it has a bit of amber hue.
If you’re wondering how many layers are good enough for a smooth finish, the answer is three to four. During application, you may also have to take some breaks as each coat takes quite some time to dry. However, no matter how many coats you think you should apply, the process will always be time-consuming.
In lighter wooden surfaces, it appears more and also gives warmth to the final shade. Cleaning oil-based products is not an easy job, but you can use mineral spirits to do so.
Polyurethane Coating: How Much Is Enough?
No matter the type of product you use, do not entirely rely on one source of information to determine how many coats should be enough for your project. However, you must know that one coat will not be enough to provide protection to the underlying wood in any case. You must put multiple coats of the product at hand.
The water-based version of your product would require more coats than the oil-based version, given that it is thinner than oil. For a general idea, you should keep in mind that if you need three to four coats for an oil-based varnish, then a water-based varnish would need four to five coats. The final decision on how much polyurethane coating you need depends on the amount of protection your surface demands.
Applying Polyurethane Coats
Polyurethane varnish can be easily applied using a spray or brush. Even though using a spray is generally recommended, it results in fumes, which can be a nuisance to someone who’s sensitive to odors.
A spray allows you to apply it in the same way you would apply paint and is a great choice for oddly shaped surfaces. This is because a spray helps you give an even coating on any shape. They may give you thinner coats, and you should always be careful as they can leave stains on your floor because of dripping. Using a spray is not a feasible choice for an amateur since you require a lot more expertise to get an even finish with it.
If your surface is flat, using a brush would be ideal to cover a huge area. It also gives the coating a thicker look, and if you need very precise results, then foam brushes should do the job.
If your surface is neither flat nor oddly shaped, wiping your polyurethane on would be the best idea. It is also a good choice for contoured and difficult surfaces. The advantage it has over spraying is that there is no dripping or an uneven finish. The con, however, is that the coat you wipe on would be thin, and you would need multiple applications.
Debunking Some Myths About Polyurethane Coating
Sometimes, misinformation can easily spread, confusing the masses. While some myths are simply senseless and cause no harm, others just prolong the solutions of problems due to their ineffectiveness. Here are some myths associated with applying polyurethane and its drying time.
Myth No. 1: Polyurethane dries faster if you thin it out using naphtha.
Some people claim that because a thinner’s evaporating time is fast, the finish will also dry faster. There are usually two steps in which polyurethane dries. The first is when the thinner evaporates, and the second is when the varnish cures completely.
In reality, if you add naphtha, it doesn’t affect anything except the fact that it can make your finish thin.
Myth No. 2: You can reduce any brush marks by slanting the panel.
In reality, doing so can cause sagging, especially if you don’t do it carefully. The procedure also does not make much sense because gravity is not reducing the brush marking; it is only evening out differences between troughs and ridges, that in turn, eliminate brush marking. If you want a finish free from brush marks, you can sand the second last coat.
Myth no. 3: Brush marks can also be reduced if you thin the solution with penetrol.
Painters often add this substance to oil paint for reducing brush marks, especially while they under sunlight or cold or hot weather. But it is not a common practice for polyurethane.
Penetrol dries slowly and can prolong the tacky stage of oil paint and polyurethane. This gives dust more time to stick to and settle on the surface. Since dust isn’t a huge problem for painters, they don’t mind it, but for furniture finishes, it is a big deal for sure. Moreover, penetrol can make your polyurethane less durable since it is an oil.
Conditions for Ideal Coating
For a perfect finish, you need to know the various steps involved in the application process. There are, however, some considerations you must make before the polyurethane coating process.
- Since most products contain VOCs, it’s important to ensure the safety of your workspace and keep it ventilated.
- Wear your protective gear such as a mask, gloves, and goggles
- Make sure to word indoors to avoid contaminants like dust.
- Make sure to have a flat surface so you can get an even coating and zero dripping.
- Ensure proper lighting and inspect your finish thoroughly for any remaining imperfections
- Clean your surface properly.
- Sand your surface well. Use coarse sandpaper first, and then proceed to use finer sandpaper. This way, your wooden surface would be even and smooth.
- Following sanding, use a vacuum to clean up any residue as it can have an impact on polyurethane adhesion.
- Raise the grain of your wooden surface by making your surface wet in case you use a water-based product.
Sealing the Surface
- Sealing your surface is important before you apply polyurethane. This is because the porosity of your wood can cause it to form bubbles.
- Make sure your polyurethane is thin. You can do that by mixing some mineral spirit with your oil-based product.
- With a water-based counterpart, use water if you wish to make the polyurethane thin.
- Use a brush to apply your sealer. It is often recommended that you use a brush with natural bristles to give your surface a smooth finish. Avoid any form of dripping while you apply it.
- To ensure good adhesion, apply your water-based product to the oil-based stain. Steel wool can also be used to roughen up the surface a bit to strengthen area bonds.
Polyurethane Coating: How to Add the First Coat
- Avoid putting the first layer of your polyurethane coating for at least 24 hours after applying the sealer. Make sure to do it only when it’s completely dry.
- Do not make this coat thin as it may not be as adhesive then.
- Smoothen out any possible drips by overlapping the brush strokes.
Applying an Oil-Based Product
Avoid using a brush with rough bristles and only use a foam one, especially if your surface is flat. If your application needs to be detailed, then brushes with bristles work well.
- Dip your brush in the can containing polyurethane and prevent the formation of bubbles.
- Try to apply thick and even coats.
- The number of coats you apply vary depending on several factors. However, three coats with sanding in between should be good enough
Applying a Water-Based Product
- Since its consistency is thinner, the application is made best with a brush or a cloth. Do not let the wood grain rise up, and make sure to apply thin coats.
- It’s not necessary to sand between polyurethane coating, but doing so won’t cause any harm.
- You must apply four coats for a busy surface, and for quiet areas, three coats are sufficient.
- Since it dries fairly quickly, you can easily apply additional coats
How to Sand the First Coat
- Let your first layer dry for at least 24 hours before sanding it. Use fine-grit sandpaper to wet sand your surface.
- Don’t let your polyurethane finish get damaged, and make sure that the sandpaper is constantly wet. Sanding should only be done to the extent of preventing an uneven surface.
- Use a wet cloth to wipe the surface down, and make sure it dries completely before applying the second coat.
How to Add More Coats
- Ensure that the underlying coat has dried completely before you add the next coat, and this goes for as many coats you wish to apply.
- If the previous coat is bumpy or uneven can you sand before adding the next one, but it’s not necessary if your surface is smooth.
- Avoid sanding the last or final coat and polish it instead to give it a glowing appearance.
- Follow the wood grain when you brush on polyurethane coats
How to Sand Between Polyurethane Coating
- Use a fine-grit material for sanding between the coating process, and before putting the next coat, make sure the last one has fully dried.
- Unlike satin finishes, when you sand polyurethane that has a glossy finish, it needs higher intensity.
- Since glossy coats can make imperfections more apparent, you must take care of it more than flatter finishes. You may also have to create an underlying layer that is rough because otherwise, your polyurethane will not adhere that well to a glossy coat.
- If you have a sanding block for your varnish, it will also need a lot of care, or it may end up causing scratches. You can also use an abrasive pad or steel wool to roughen up your surface, but they will not be as good as sanding.
How to Polish the Surface
- To get the best end result, polish the final polyurethane coat, especially if you wet sanded the second coating.
- However, before applying polish, ensure that the final layer has cured properly. You must give it 48 hours to cure, though.
- The polishing compound or paste you use should be of good quality with fine abrasiveness so that no scratches remain as a result of the sanding paper. It will smooth out your surface and make it glow. To apply the paste to your surface, rub a damp cloth in circular motions.
- Before buffing your surface with a dry cloth, make sure that the surface is completely dry first. If you think your surface is cloudy, apply some more paste, following which you can buff it. You can keep doing it till you get the desired result.
Tips for Using Polyurethane
- If you want to avoid forming bubbles, do not shake your polyurethane liquid. Instead, stir it.
- Use a high-quality brush with bristles to avoid marks and bubbles, especially if you’re applying oil-based polyurethane.
- Use a synthetic brush for a water-based product because using a roller brush may cause bubbling.
- If you need the polyurethane coating to dry quickly, you can thin it out before applying.
- Avoid thinning the first coat, though, and only do it for other layers because it can have an impact on how well your varnish can stick to the surface.
- Use a solvent to keep your brush damp when you use it to apply polyurethane. The brush can easily be cleaned by dipping it into water or spirit.
- Do not wipe the brush on the polyurethane can as it may cause bubbles. Instead, just tap it away.
- Use polyurethane on any visible pores on the wood if you wish to have an even surface.
- You can avoid dripping by applying thinner coats, and such coats can be achieved by wiping or spraying. In case you see any drips, use a sharp razor to remove them.
Going ahead with a DIY project requires you to have knowledge about the coating process. Nonetheless, polyurethane coating has many advantages for wooden surfaces, but you must understand that the method of application can slightly differ due to the different polyurethane types available in the market. Therefore, we hope that you will have a sufficient amount of understanding about applying polyurethane coats to your surface through this guide.
Frequently Asked Questions
What Should Be the Ideal Temperature for Drying Polyurethane?
Airflow and humidity determine what temperature is suitable and how long the drying time could be. For example, 75 to 77 degrees Fahrenheit is a good temperature with 50 % humidity to get the desired results.
On Average, How Long Should Polyurethane Take to Dry?
Due to thin consistency, water-based items tend to dry quickly, often in six to seven hours. However, it takes about a day for oil-based items to dry. Again, the time it takes to dry depends on many factors like ventilation, temperature, layer thickness, etc.
Is There a Way to Avoid Air Bubbles in My Polyurethane Finish?
Bubbles can be removed by wiping them with mineral spirits on a piece of cloth. If you do it for fifteen to twenty minutes, you can remove the bubbles without any damage to the surface. If you want to avoid risks, simply wait it out till the finish is completely dry and then remove the bubbles by sanding. If you wish to avoid the formation of bubbles from the beginning, then thin out your polyurethane layer with mineral spirits or water while applying.
What Do I Do if There Are Brush Marks in My Polyurethane Finish?
You can deal with brush marks by sanding using fine sandpaper. First, however, make sure that it is done evenly.Clean up and remaining residue or dust following that. Mostly, thick coats are more susceptible to brush marks as compared to thin coats.