Most likely, you have worked with polyurethane in your previous woodworking projects. The formula is among the most versatile finishes and is available in various forms. But most users wonder if they can use the product to revive old varnish. So, let’s answer the query, Can You Put Polyurethane Over Old Varnish?
Yes! It is okay to put polyurethane over old varnish without ruining the finish. Moreover, the formula delivers a long-lasting and hard-wearing finish with a high resin value. Therefore, you can expect the surface to withstand harsh and constantly changing weather conditions.
Nonetheless, please be careful on how and where you apply the formula. For instance, some kinds of polyurethane and varnish do not yield results on certain wood types. Also, it would be prudent to read through this for more instructions on working with these products.
Can You Varnish Over Old Varnish
It is possible to varnish over old varnish, but please avoid making it a daily practice. Varnishes are often build-up coats of various chemicals and create a barrier between the new formula and the air. Thus, the new varnish layer may not adhere well to the underlying surface.
In addition, there is a likelihood that the new varnish coat will flake or peel within a short duration. So, it would be better to remove the old varnish if you want to achieve a quality finish.
Generally, it is advisable to completely remove existing waxes, oils, dust, varnish, grease, sticky stuff, and uneven areas before applying any formula. This way, you’ll facilitate a tighter bond between the surface and the topcoat.
Can You Recoat Polyurethane Without Sanding?
You can recoat polyurethane without sanding and still deliver a beautiful finish. Also, you are safer using a high-quality foam brush. It will not leave any streak marks or spots that will require sanding.
Even so, remember that polyurethane will deliver a rough texture if you do not want sand between coats. Fortunately, the naked eye does not notice the difference, and the layers will adhere together tightly.
What Grit Sandpaper Do I Use Between Coats of Polyurethane?
It is best to use 240-grit sandpaper to sand between polyurethane coats. However, please allow the last coat to dry for about 24 hours. This way, the finish will be strong enough to withstand sanding.
Can You Put Polyurethane Over Old Lacquer?
No! The polyurethane coat will not bond with the lacquer finish. In addition, you cannot even use them interchangeably, and the surface will bubble up and eventually flake, leaving an ugly finish.
It would be better to sand off all the existing lacquer on the surface before applying a polyurethane coat. Also, please consider the wood in question as some wood types and polyurethane formulas are not compatible.
Lacquer comes from dissolving nitrocellulose, pigments, and plasticizers in volatile solid solvents. You can also make it with a shellac solution in denatured alcohol and use it to mimic expensive wood finishes like mahogany, cherry, and walnut.
Additionally, a lacquer finish is shiny and smooth and protects the surface from abrasions and scratches from heavy traffic. Even better, it keeps the lumber safe from harmful outdoor elements such as rain, snow, and heat.
On top of that, the formula creates an excellent barrier against ultraviolet rays. Hence, the wood retains its original color for a longer duration.
That said, I understand why you’d want to add a polyurethane coat on an old lacquer surface. It sounds like a perfect way to protect your wood. But as we have seen, the practice will lead to a peeled and flaked project. So, it is not worth it!
Can You Put Water Based Varnish Over Oil-Based Varnish?
It is okay to put water-based varnish over oil-based varnish. But there is a caveat. Ensure that the finish is in good condition before you add the topcoat. Otherwise, the new varnish layer will not adhere to the surface.
Fortunately, we have wood prep products, cleaners, de-glosser, synthetic steel wood, and mineral spirits to help remove dirt, grease, and wax on the surface. So, you can easily get started on the project.
On top of that, it would be better to allow the underlying finish to always dry well before adding another coat. Also, generally, you need about 72 hours for the surface to cure to the touch. So, you won’t be waiting a long time.
What Is the Difference Between Spar Urethane and Polyurethane?
The main difference between spar urethane and polyurethane is that spar urethane is more vulnerable to chemical and alcohol damage. Even worse, wood cleaning chemicals can damage the finish.
On the other hand, polyurethane can withstand most chemicals, including alcohol. Hence, it works pretty well with wood cleaners. And you can clean it regularly to maintain its original shine.
There are many other differences between these formulas. However, it would be best to look at each product. This way, we’ll understand them better, making it easier to compare and contrast them.
This formula is popular in multiple woodworking and painting workshops. More so, it is available in various forms, making it suitable for multiple projects. For example, you can choose from water-based or oil-based polyurethane.
In addition, you will get an impressive assortment of gloss levels for your household and commercial applications. Better still, polyurethane delivers a long-lasting final coat for wood surfaces.
On top of that, the product is a transparent synthetic resin and resists abrasion. It also resists fungus, water, and mildew to keep the wood integrity intact for a long, long time. But it would be better to use a water-based polyurethane formula for indoor applications as it has fewer toxic fumes.
- Spar Urethane
Spar urethane has a high ratio of resins compared to solvents. It also has some added compounds that help it to disperse ultraviolet light excellently. Therefore, you will commonly find it in constructing outdoor furniture or wood that deals with excess sunlight.
Please note that spar urethane is not as common as polyurethane as it is most useful in the marine field. The finish withstands elements on wooden poles that support boat sails or spars of a ship.
On top of that, spar urethane does not feature a much thicker consistency than polyurethane. More so, it dries through oxidation, which makes it cure much faster with enough air circulation.
That said, let us get into a detailed account of the difference between the above formulas.
- Dry Times
Please note that there are two different drying techniques to consider when contrasting polyurethane and spar urethane. For example, polyurethane requires more time to dry, and high humidity or cold air can drastically slow down the process.
This scenario is more disadvantageous as you risk dirt and dust getting in the finish before it dries. In addition, you’ll have to clean or even sand the surface during a later recoat.
In contrast, spar urethane dries by exposure to airflow or oxidation. Hence, you can easily speed up the finishing process by setting a fan on your complete project. However, this process may shorten your work time, making it harder to deliver a smooth finish.
Fortunately, this drying strategy allows you to complete your project more quickly, especially when you need multiple coats to deliver a sophisticated finish. On the other hand, polyurethane only allows you to do two coats each day. So, you have a slower work time.
It is prudent to use a brush to apply both spar varnish and polyurethane. However, please consider a bristled brush for spar urethane because of its thicker consistency. Also, get a foam brush or a traditional brush for polyurethane formulas. This way, it is easy to get a more even and uniform layer.
Unfortunately, it is pretty easy to leave brush strokes when brushing spar urethane on wood. Worse still, a lot of air circulation causes the finish to dry extremely quickly, making it hard to remove the strokes.
Therefore, it would be better to pay close attention during the brushing or application stage. This way, you will deliver a smooth finish from the beginning and have fewer mistakes to correct later.
On the other hand, polyurethane delivers a longer application time. Hence, it is possible to go back and remove any brushstrokes and imperfections. In addition, the formula is much thinner than spar urethane and does not run easily.
It is okay to apply the products using the wiping technique. However, you are required to dilute spar urethane with mineral spirits. Please consider a ratio of one to one for a better result.
Next, use a clean, lint-free cloth for the application to avoid leaving cloth fiber on the surface. Also, please note that you may need to add more layers because of the dilution. Or you risk not getting a consistent finish.
Alternatively, you can spray spar urethane and polyurethane on the project. This way, you’ll reduce the risk of brushstrokes that might ruin the finish. In addition, polyurethane’s thin qualities make it perfect for spraying.
Interestingly, you will get polyurethane in spray cans ready for use, saving you on the clean-up that the traditional paint sprayer requires. However, please note that you might pay a few more dollars for this added convenience.
On the other hand, experts believe that spar urethane is quite thick, and it is best to thin it with mineral spirits before spraying. Also, you will need to apply more coats to achieve satisfactory consistency and thickness.
Ultimately, all you need is a smooth surface free from drips and bubbles, regardless of the chosen finish. Therefore, ensure that there is no dust, sand, or dirt on the surface before application. Otherwise, you will trap particles under the finish and ruin it.
- Oil Content
This attribute is probably among the most common differences you will observe. Spar urethanes usually have more oil. Hence, they are softer on the wood, and you’ll have an easy application time.
Even better, this extra oil delivers a more flexible finish. Thus, the wood naturally contracts and expands with temperature and humidity changes, decreasing wood cracking and chipping.
Conversely, polyurethane has a lesser oil content and is less pliable. Also, although it can hold up to some wood movement, you will find it far less flexible than its counterpart. Thus, the wood may crack or chip in extreme environmental changes.
- Indoor or Outdoor Use
Originally, spar urethane was for the wooden parts of ships to keep rot and decay away. But now you will find it on wooden furniture and household products. Fortunately, it is a safe product and offers numerous benefits.
For example, the product gives the user a little more flexibility. Hence, you can use the wooden items as you deem fit. In addition, it has a pretty fast drying and curing time. So, you are free to use the surface within a very short duration.
On the other hand, oil-based polyurethane is usually a more durable option. In addition, it can withstand extreme weather changes. And even better, you can use it for both interior and exterior applications.
Like spar urethane, oil-based polyurethane is also water-resistant. Better still, it can withstand rain and snowstorms. So, you do not have to panic any time there is a downpour. Your finish is safe!
On top of that, spar urethane has additives that protect the wood from harmful UV rays. These compounds also help the wood keep its original shade even after excess sunlight exposure.
Conversely, polyurethane does not have any of the above additives. It will eventually fade or change color. Therefore, it will adopt a more amber color over time and even allow the wood to bleach.
Fortunately, both polyurethane and spar urethane products are flexible enough to allow the lumber to contract and expand. Therefore, the wood will move naturally with temperature and humidity levels, regardless of what you choose.
Oil-based polyurethane and spar urethane have some fumes. Hence, you are safer if you work in a well-ventilated area. In addition, please consider using these products away from fire and heat to avoid fire hazards.
Please read the manufacturer’s application instructions as some products may need you to wear a respirator mask. Also, polyurethane and spar urethane formulas are not the most eco-friendly products in the market. Therefore, it would be best to protect yourself.
Get gloves and some protective clothing to avoid running your favorite pair of jeans or t-shirt. In addition, consider working with water-based products when working in poorly ventilated areas.
- Exposure to Chemicals
Spar urethane and polyurethane reach differently to chemicals, even those in wood cleaners. For example, spar urethane is prone to damage from alcohol and other wood cleaning products. Even worse, the finish may eventually break down.
Conversely, polyurethane is more accommodative and does not degrade with alcohol use. More so, it is perfect for tabletops or bar counters that undergo routine cleaning with wood cleaners.
These products share multiple similarities. Hence, their prices may be the deciding factor. But first, please note that spar urethane varies significantly from one manufacturer to another. However, you will find it to be more pricey from high-quality brands.
On the other hand, polyurethane products are more affordable to customers. But you can expect to chuck out some more coins when you purchase them in ready-to-use spray containers. Fortunately, the cost is worth it as the application formula is easier and more convenient.
That said, both spar urethane and polyurethane products are perfect choices for your work. Thus, it would be prudent to consider how much time you have to work on the surface and how you intend to use it. This way, you’ll get the best formula for your work.
How Do You Apply Polyurethane Without Brush Marks?
Polyurethane forms a seal on the surface and gives it superior protection from elements. Also, you can apply it on metal, wood, or a paint layer as it is versatile and useful for multiple projects.
Unfortunately, it is very easy to leave brush marks on the finish when using a paintbrush. But you can always avoid these application issues with the correct formula consistency and thickness.
In addition, it is possible to deliver a smooth and flawless polyurethane if you adhere to the proper application procedures. Even better, the ridges resolve themselves and leave an even finish.
So, the first step is to get the required supplies for the job. For example, you’ll need a high-quality foam paintbrush, paint thinner, a bristle paintbrush, a lambswool applicator, and a paint stir stick. Then, follow the manufacturer’s application guidelines.
However, sometimes the brush marks do not resolve after the allocated time. Fortunately, we have some simple corrective techniques to restore your perfect finish.
Check them out!
- Use natural-bristle brushes instead of synthetic ones as they leave fewer marks. In addition, natural-bristle brushes are the most suitable for oil-based polyurethane formulas.
- Get a foam brush or a lambswool applicator. This way, you will avoid brush marks that natural-bristle brushes or synthetic-bristle brushes would deliver.
- Please avoid dragging the applicator back and forth in quick motions during application. Otherwise, you will create more bubbles on the finish and compromise the result. In addition, the exercise may cause the formula to dry faster, giving the brush marks less time to level out.
- Thin the polyurethane with some paint thinner and blend it with a paint stir stick. Remember that brush marks hold their shape and refuse to settle out when the formula is too thick. Therefore, thinning helps to deliver a smoother surface.
- Experiment with various amounts of paint thinner until you get what works best for your work. But generally, most construction workers and homeowners are okay with an equal amount of thinner.
- Allow the first polyurethane layer to dry overnight before sanding. Then, use 280-grit sandpaper for the job. Also, apply successive polyurethane coats and sand each application as it dries.
- Use 600-grit sandpaper after the final coat to deliver a more sleek and smooth surface.
NB: Please work in a well-ventilated area to keep the room from being too hot or dry. This way, you’ll allow the brush marks to resolve on their own.
Frequently Asked Questions
- Why Is My Polyurethane Still Sticky?
You will find this problem mostly with oil-based polyurethane formulas as they dry in various stages. First, the solvent will evaporate and leave the resin behind, which requires some time.
Unfortunately, this duration lengthens or shortens depending on the thickness of the finish and humidity or temperature levels. So, you will observe a sticky finish when the solvent evaporates.
- What Will Polyurethane Not Stick To?
Polyurethane does not stick to polyethylene, polypropylene, or substances like silicon and Teflon. Therefore, please check your surface before applying the formula to avoid wasting time and resources.
On the other hand, polyurethane works perfectly for multiple materials, whether normal, porous, or non-porous. In addition, you can use it for metals, curved epoxy, glass, tile, wood, leather, brick, plastic, and concrete.
- Can You Put Polyurethane on Too Thick?
Yes, it is possible to have a thick polyurethane coat. But you will need to give it more time to dry, especially if the weather is too humid or cold.
Even so, it would be best to have multiple thin layers than one thick one. This way, you are sure of a more consistent and uniform finish. Even better, multiple coats allow you to correct brush strokes and other imperfections.
Polyurethane is a perfect product for multiple woodworking projects. More so, you can choose from water-based or oil-based polyurethane formulas and a wide range of gloss levels. In addition, the product delivers a durable topcoat for lumber surfaces. But there is a rising concern among woodworkers:
Can You Put Polyurethane Over Old Varnish?
The answer is yes. Applying polyurethane over varnish is a good idea. Varnish keeps wood surfaces clean and presentable, while polyurethane helps maintain the object’s durability. Therefore, you kill two birds with one stone!