There are many clear protective coating products on the market, formulated from numerous polymers, solvents, and additives. They are often classified as a shellac, varnish, polyurethane, or polycrylic. While these terms are commonly used to reference a final finish in general, these products are not the same, nor should they be used interchangeably.
Shellac is a natural product (made from the secretions of the lac beetle) that is available in clear, amber, and pigmented (white) formulations, although clear shellac is the favored choice as it is non-toxic, food safe, and safe to use on children’s items once dried and hardened. On existing pieces of furniture, it is used to block tannins, stains, and odors before painting. It can also be used to protect new finishes, and very fine furniture is often finished with clear shellac using a technique known as “French polish.” Shellac can be affected by heat (i.e., leaving white rings from a hot mug) or household chemicals, so a kitchen tabletop might not be the best place to use it for a final protective coat.
Essentially a plastic in the form of a liquid until it dries, polyurethane is available in both water- and oil-based formulations and comes in sheen varieties from dead flat to gloss.
Water-based polyurethane, known as polycrylic, is popular because of its low odor, low toxicity, and easy clean up. It goes on clear without adding a slight yellow color and dries fast. As with shellac, a polycrylic does not hold up well to heat and household chemicals. It’s good for surfaces that won't be exposed to heavy wear and tear.
Polycrylics do not always work well over matte paints. The additives in a matte paint keep the polycrylic from drying at its normal rate, often leaving behind cracks once dried. It can be difficult to use on larger pieces because it dries quickly. A polycrylic should always be applied in very thin coats; over darker colors it can give a milky finish if applied too thickly. Once you have applied a coat, you should avoid going back with a brush or roller to smooth as this can make your surface a rather untidy mess.
Oil-based polyurethanes are slightly more durable than their water-based counterpart, especially when it comes to handling heat. It will create a yellow or amber hue, especially to light colors. When working with a polyurethane, use a respirator in a well-ventilated area; polyurethane is highly flammable and toxic. Care should be taken with the disposal of leftover materials and rags. Polyurethane takes much longer to dry and cure than a polycrylic, so plan accordingly and follow the manufacturer's directions.
Varnish is used often in wood finishing but also looks beautiful over painted finishes. Varnish finishes produce sheens from dead flat to semi-gloss (through the addition of "flatting agents”) to gloss. There are two formulations of varnish available, oil-based and water-based.
Oil-based varnishes are often applied over wood stains as a final step to achieve a film for sheen and protection. They are thought to give wood more depth and color because they are actually absorbed into the wood surface, while the water-based version dries on the surface. Oil-based varnishes take longer to dry (as long as 24 hours for each coat) and several days to fully cure. They are not as popular as water-based varnishes because they are somewhat difficult to apply, and they have an odor.
Spar varnish, a form of oil-based varnish, is good for outdoor projects and for raw wood used for exterior doors and trim on rustic homes. In addition to protecting the wood, it also provides natural ultraviolet light protection.
As with other oil-based products, use a respirator and apply in a well-ventilated area. Care should be taken with the disposal of leftover materials and rags. Always follow the manufacturer’s directions.
Water-based varnishes, on the other hand, are easier to apply, dry quickly, and have very little odor. When applied over raw wood, grain raising may happen that requires you to sand after the first coat with fine grit sandpaper. For water-based varnishes, hardening or “curing” takes place as soon as the water has fully evaporated. They are typically more expensive than their oil-based counterparts. Water-based varnishes are good for commercial/high traffic areas where durability is important.
Use the following information when working with any clear protective coating product:
Before applying make sure your surface is clean and free of dust and other surface debris.
Stir the contents well before and during use. For containers that do not allow space for a stir stick, gently roll the container in your hands to mix or empty the contents into a bucket and stir gently. Avoid shaking the container. This can cause the formation of tiny bubbles that can dry into the final finish.
For larger projects, batch your cans together to ensure a consistent sheen level.
Allow your paint finish to initially dry a minimum of 24 hours before application, but always check the manufacturer’s directions as this time can vary widely.
Keep in mind that the glossier the sheen, the more obvious any surface flaws will be due to the increased light reflectivity. Conversely, if you are working on a textured surface, the sheen will not be as reflective due to light being “absorbed” by the texture.
Additional flatting agents present in all sheens other than gloss are generally the reason for a milky appearance (called “blushing” or “frosting”) when satin and matte clear coats are applied too thickly over darker colors. This is especially true where a double thickness is applied, such as in areas that are cut in with a brush and then overlapped with a roller. An excellent industry trick to avoid this is to start with gloss for your first layer and then work down to your desired sheen using satin or matte for the final layers. This will result in a much clearer finish.
Temperature is important to the success of any clear coat. If the room temperature or surface temperature is too cool (55°F or 13°C and below), you might end up with “fisheyes” or pinholes in your finish. Cooler temperatures may also cause problems with how the finish hardens or levels out. On the other hand, if the temperatures are too hot (85°F or 30°C and above), the finish may dry too fast, which could prevent it from leveling out because the water content evaporates too quickly.
Humidity is also a factor. If the humidity is too high, the finish may not dry properly, and trapped moisture could cause a cloudy appearance. In general, it is not a good idea to rush drying a clear coat as this could cause a skin to form, trapping the moisture and not allowing it to evaporate at a normal rate. It’s also wise to avoid blow dryers, fans, or air conditioning and/or heat vents blowing directly on a freshly applied clear coat.
Water-based clear coat products may be thinned with a small amount of water (about 10%) for easier application and good flow out. However, if you thin your products too much, you may experience fish eyes or pinholes in the finish. Always test on a sample first.
To reduce brush or lap marks when applying a clear coat, use a quality brush. Apply thin coats and do not over brush or back brush as the product is setting. Trying to touch up or smooth an area that has already begun to dry could cause “flashing” (an uneven sheen that reads as spots or streaks). Working with speed and confidence is key, as well as applying at least two coats to help eliminate any skips and to even out the finish.
Allow each coat to dry completely before applying the next.
Before wet sanding and polishing any clear coat finish, it is recommended to let the finish dry a minimum of 72 hours.
Remember that no clear protective coating will be a “coat of steel.” These finishes deserve respect.
It is highly recommended that you spend some time experimenting with different clear coat products to test their compatibility with your underlying materials and to get a feel for how they brush, roll, sand, and polish. Always follow the manufacturer’s directions. Read the labels carefully and, if needed, consult with the manufacturer to learn more about their product, including its formulation, the number of coats required, dry time, and cure time.