Answers to pro painters’ most pressing questions by Rick Watson
Can you help me understand the differences between conversion varnish, two-part polyurethanes, lacquers, and pre-cat lacquers?
Nitrocellulose lacquer is the traditional topcoat of choice for wood finishing. Some of the benefits of lacquers include a rich appearance, easy application, and the ability to easily touch up or correct flaws in the coating. Each layer of nitrocellulose lacquer “melts” into the previous film, resulting in an even and uniform appearance.
Historically, the limitation with nitrocellulose lacquer has been a tendency to yellow and a relative lack of durability for heavy-wear applications. New technology, including the addition of UV absorbers and higher solids formulations, have resulted in high performance lacquers.
For higher quality applications, look for a catalyzed finish, especially one that is high build and/or contains UV absorbers. CAB acrylic clear is a type of nitrocellulose lacquer that has better yellowing resistance than most nitrocellulose lacquers.
CAB acrylic pigmented lacquers
CAB acrylic lacquers are designed for interior wood finishing where light stable, non-yellowing whites and pastels are required. These white lacquers can be used for finishing kitchen cabinetry, furniture, and other interior woodworking. Cellulose Acetate Butyrate (CAB) acrylic represents the best chemistry of lacquers for resistance to yellowing.
High-performance pigmented conversion lacquers are for the general wood finishing market. After catalyzation, they can provide up to six months pot life as a white pre-cat lacquer. Many meet the KCMA specifications as a self-sealed system or over several catalyzed vinyl sealers. Some of these pre-cat Lacquers contain a UV absorber for improved resistance to yellowing. Catalyzed lacquers offer improved hardness, and better chemical and moisture resistance.
Waterborne acrylic topcoats
Clear and pigmented waterborne acrylic topcoats are gaining in both performance and popularity with the increased interest in sustainability and green coatings. Waterborne acrylic technology can provide durability as well as moisture and chemical resistance. Recent advances in waterborne technology have resulted in a final appearance that matches the traditional look of lacquer, eliminating the biggest resistance to waterborne products. Waterborne acrylics are ideal products to help meet many VOC regulations and support efforts toward lowering the environmental footprint.
Limitations in waterborne acrylic technology can include differences in the application process, grain raising, and more difficulty in touching up or correcting worn or damaged areas of the coating. Waterborne acrylic topcoats are often used for non-shop application in VOC-regulated areas, or for finishers with environmentally focused customers.
Conversion varnish is an extremely tough and durable catalyzed coating, and should be the product of choice for the most demanding end uses and customers. It is one of the best in moisture and chemical resistance of any wood finish. It also has a crystal clear option, making it an exceptionally good product choice for use over white, pickled, or light-colored stains. Pigmented conversion varnish also has good resistance to yellowing.
Conversion varnish has a multi-phase curing mechanism, requiring both solvent evaporation and catalyzation, so it takes longer than traditional wood finishing topcoats to fully harden. Thus, conversion varnish is not recommended for application in facilities where ambient dust and debris are present and could blemish the coating.
Conversion varnish is most popular with finishers of institutional furniture for offices, schools, and churches and is also used for high-end cabinetry packages, vanities and other high-quality custom wood applications.
Most large kitchen and bath cabinet manufacturers prefer to use conversion varnish. Smaller shops that can’t climate control their shop environment 24/7 should not consider conversion varnish systems. Conversion varnishes are the workhorse for the kitchen cabinet industry and are widely used on office furniture and interior wood surfaces where high performance is required, such as store fixtures.
Clear and pigmented polyurethane finishes are popular in certain parts of North America, particularly for on-site finishing. Polyurethane is similar to acrylic technology in both durability and appearance – uniform, with lack of depth and luster. Repair is more difficult than traditional lacquer technologies. Many polyurethanes have more forgiving application characteristics, which is why it is popular for on-site finishers and application.
Urethanes offer excellent film properties and the added benefit of containing no formaldehydes which are becoming more and more restricted in use. Urethanes would commonly be found on office furniture and lab equipment that would be potentially exposed to harsh chemicals.
UV coatings – clear and pigmented – offer many benefits such as excellent chemical resistance, excellent film properties and appearance. You will find the majority of UV applications are done on what is referred to as a “flat-line” finishing line. They’re commonly used for kitchen cabinets, office furniture, entry doors and other markets where high volume and speed in finishing is required.
Clear and pigmented polyesters are extremely hard, high-build finishes. Polyester coatings are specialty coatings popular for high-end furnishings and musical instruments like guitars, organs, pianos and other situations where a high gloss wet look is required. Think of that bar-top finish with the coins embedded in the coating.
This article was originally published in the Winter 2018 issue of PPC magazine. Rick Watson, director of Product Information and Technical Services at Sherwin-Williams, answers questions from pro painters in the Ask Your ProPartner™ column in PPC magazine.
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