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 combining a secretion from the female lac bug with an alcohol-based solvent) that is very safe once dried and hardened. In addition to adding a protective coat, it can add a warm amber color to wood. Very fine furniture is often finished with shellac using a technique known as “French polish.” It can be affected by heat (i.e., leaving white rings from a hot mug) or household chemicals, so a kitchen table might not be the best place to use it.
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 satin to gloss.
Water-based polyurethane, or polycrylic, is popular because of its low odor, low toxicity, and easy clean up. It goes on clear without adding a slight color that oil-based versions can, and it dries fast. As with shellac, water-based polyurethane 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 work well over standard latex paints but not over matte paints. The additives in a matte paint keep the polycrylic from drying at its normal rate, often leaving behind cracks once dried. They are often difficult to use on larger pieces because they dry quickly. Due to its runny consistency, you will have to apply very thin coats on vertical surfaces and watch closely for drips. Once you have applied a coat, you can’t go back to re-do it as it will make your surface an untidy sticky mess. Over dark colored paints they can give a milky finish if applied too thickly.
Oil-based polyurethane is slightly more durable than its 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 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 water-based, so plan accordingly and follow the manufacturer's directions.
The name of this finish is often used generically for a finish or topcoat. Varnish is a hard, thin, transparent protective finish that is used often in wood finishing. Varnish finishes are usually glossy but may be designed to produce dead flat, matte, satin, or semi-gloss sheens by the addition of "flatting" agents. 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. Annie Sloan Lacquer is a water-based varnish. When applied over raw wood, grain raising may happen that requires you to sand 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 application guidelines when working with any clear protective coating product:
Before applying any clear coat product, make sure your surface is clean and free of dust and other surface debris.
Avoid shaking the container. Shaking can cause the formation of tiny bubbles that can dry into the final finish.
On larger projects, batch your cans together to ensure an even sheen level.
If you are applying a clear coat product by brush or roller, allow the paint finish to initially dry 24 - 48 hours before application. Depending on drying conditions and the materials used to create the finish, this time can vary widely.
Test the different sheens of your clear coat products. 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.
The normal sheen of a clear coat product is gloss. Flattening agents are added in to make it satin or matte. This means these products will have more solid particles in them and, therefore, should be stirred thoroughly 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.
Additional solid particles 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” of the product is applied, such as in areas that are cut in with a brush and then overlapped with a roller.
An excellent industry trick when applying multiple coats of a clear coat product 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 finish. If the room temperature or surface temperature is too cool (55°F or 13°C and below), you might end up with “fish eyes” 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 product, use a quality varnish 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.
Before wet sanding and polishing any clear coat finish, it is recommended to let the finish dry a minimum of 72 hours.
Allow each coat to dry properly before applying the next.
Most water-based clear coat products have a cure time of approximately 30 days, depending upon the thickness of application and environmental conditions.
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 the different clear coat products to test their compatibility with your underlying materials, and to get a feel for how they spray, 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.