Wood Sealer vs. Polyurethane


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On completion of wooden objects or structures, you need to apply a protective coating. We call this protective coating finish. Applying finish to wood can be easy or difficult depending on the type of finish you apply. In this post, we discuss polyurethane as a wood sealer and other sealers.

We apply a sealer to wood to protect it from damage. Wood sealer can be oil, paint shellac, and even stain. It provides a layer of protection to the wood and sometimes prevents the grain from rising. You can apply a sealer to add color or depth to the wood. It is a common practice to apply a sealer to stained wood to prevent bleeding.

For best results, after staining wood, you can sand it down and apply a coat of thinned shellac as a sealer. You can also use a sanding sealer or any other commercially available sealer. You should not use shellac if you have used a non-grain raising (NGR) stain or water-based stain.

Using Polyurethane as a Sealer

Lacquering wood floors. Worker uses a roller to coating floors.

If you want to use polyurethane as a sealer, then you need to ensure that the grade of poly that you use is compatible with the stain. Although polyurethane is often confused as being different from wood sealer, in reality, it is a type of sealer.

You need to treat polyurethane carefully to get the best results. Always pay attention to the labels. Some types of poly require a particular thinner or the manufacturer may recommend a particular way of application. While finishing wood, the varnish and the sealer must be compatible with each other, failing which you will not get the best results. Applying poly as a final sealer coat imparts a smoother finish to the wood.

You will find oil-based polyurethane the most durable and hard-curing of finishes. It also bonds well to itself making the successive coats easy to apply. It is always a good idea to use fine-grit sandpaper to sand down the previous coat after drying, before applying the next one. Sanding between coats helps to get rid of dust nibs.

Polyurethane doesn’t adhere to commercial sealers though, particularly sanding sealers. But you can use non-poly urethane varnishes as a sealer for such finishes. You can use shellac for sealing wood below polyurethane, but normally, poly is the preferred choice.

However, you may find first applying a coat of shellac useful when refinishing old furniture, because shellac is good for covering old stains and odors. Always use dewaxed shellac which you can buy commercially from home centers. It comes in liquid form or also as flakes which you can dissolve in denatured alcohol.

How to Prepare a Traditional Sealer

If you have the time and patience, perhaps you would like to try this traditional sealer that uses lacquer and shellac. Take one part of white shellac (4-pound cut) and add to it three to four parts of denatured alcohol. You can use this mixture for refinishing jobs, but you won’t find it suitable for use with polyurethane varnish, water-based stains, or NGR stains.

Another option for preparing a natural varnish or lacquer finish is to mix natural varnish or lacquer in a ratio of 1:1 with turpentine or mineral spirits. Then to make the lacquer sealer, you can thin down the mixture with turpentine or mineral spirits. You cannot use this sealer with poly or shellac varnish.

Polyurethane Vs. Varnish as a Sealer

Room Of House with Finished Wood Floors.

As a DIYer, one of the biggest home improvement projects is applying a finish to hardwood flooring. Two of the most popular options to finish hardwood floors are polyurethane and varnish. Although we tend to club both of these finishes together they have varying degrees of durability and appeal and should be dealt with separately.  You need to make certain considerations based on your requirements.

Durability

How long do you need your flooring to last before needing another coat? You have to consider the purpose of your flooring. If it is going to be subjected to heavy traffic, you can use an oil-based polyurethane finish. You can apply water-based poly to the flooring of a room with less traffic like a guest room or an attic.

Hazard Levels

Some finishes catch fire easily and have more chemicals. If you have children and pets, or a poorly ventilated room, water-based polyurethane is a good choice.

Color

Do you want your floor to be the color of the base wood or would you like to have a tint or special effects? There is a variety of options available in the market, so you can choose accordingly.

Type of Finish

Wood selection

Depending on your skill level, you need to choose the type of finish to use. For example, if you are a newbie, then a water-based poly would serve the purpose. On the other hand, if you have a certain degree of experience, you can use oil-based poly or some type of composite finish with a more complex method of application.

Drying Factor

How quickly do you need the coat to dry? Some finishes will dry overnight to 24 hours, while others can take days to dry. Be aware of the drying time when you purchase your finish.

Budget considerations

Different brands come at different prices. Then, the cost will largely depend on the type of finish you buy. Oil-based poly is among the cheapest finishes followed by water-based poly and varnish being the most expensive. You will most likely have a fixed budget, so you need to select your finish accordingly.

Polyurethane as a Sealer

Lacquering wood floors. Worker uses a roller to coating floors.

For hardwood floors, polyurethane is better than varnish. Oil-based poly will provide you with a rich color with even a single coat. Water-based poly will give you a more natural look with less odor. However, in the long run, oil-based polyurethane is more durable and protects the wood better against heat. But if water-based polyurethane dries quickly, so it is a good option if you are in a hurry.

Varnish as a Sealer

a wooden surface half coated with protective varnish for wood

In contrast to polyurethane, varnish is seldom used indoors. It is well-suited for finishing decks, fences, and outdoor furniture.

Varnish contains a high concentration of solids, making it more resistant to UV radiation from sunlight. The resin, oil, and solvents in varnish also add to its durability.

Varnish has lower toxicity, higher resistance to UV radiation but you need a high degree of skill to apply it. If mixes or applied incorrectly, the varnish is prone to bubbling, peeling, or cracking.

Conclusion

As an indoor wood sealer, the best choice is polyurethane which you can use for finishing indoor projects like cabinets, bookshelves, desks, and various types of furniture. Varnish, on the other hand, serves best for outdoor applications.

When deciding on which finish to use, you have to consider the variables that we have discussed here. If you choose carefully, you will always be successful in making the best-looking woodworking projects around.

Happy Woodworking!