When I was first introduced to nitrocellulose lacquer and spray finishing, lacquer was commonly defined as an evaporative coating that did not significantly change over an extended period of time. It did not go through a chemical process giving it properties significantly better than existed when most or all of the solvents had left the film. Therefore, it could be dissolved in the same or similar solvents.
In contrast, a varnish — composed of resin and oil — loses its solvents by evaporation and then cures by crosslinking with oxygen. It could not be dissolved by adding solvent.
Enamels were varnishes that were colored by the addition of pigment, but the definition was (and is) imprecise and seems to be applied to paints at the manufacturer’s whim or marketing intent. The term “lacquer enamel” further confuses the issue.
Catalyzed coatings were, and remain, film formers that cure by the agency of a catalyst — such as an acid — added to the coating shortly before application. The catalyst sets off a chemical reaction in the film, creating a bond between polymer chains. The cured coating is also not dissolved with solvent.
A catalyzed lacquer is a coating that dries by evaporation, acting very much like a straight lacquer (defined above with no chemical curing or crosslinking) and then cures by the reaction initiated by the catalyst. Catalyzed lacquers further shifted the definition of lacquer.
Conversion varnish is a post-catalyzed coating with good spraying properties, excellent durability and resistance to heat and moisture. But it has a short pot life, doesn’t build as well as other finishes and is difficult to prepare.
Until the early 1990s, uncatalyzed straight nitrocellulose and acrylic lacquers were the standard for wood finishing. The industry struggled to produce solvent and waterborne coatings that were as good in an effort to reduce lacquer’s undesirable characteristics. The results were blends of nitrocellulose and urethane; cellulose, acetate and butyrate; and modified acrylic blends.
Pre-catalyzed lacquers are now becoming the new standard as the market seeks better performance without a great increase in price or complexity.
Pre-cats earn their stripes
The first pre-catalyzed lacquers (we’ll call them pre-cats for short) had some issues with shelf life and the recoat window, the period of time before crosslinking has proceeded too far to allow safe recoating. After that time, the applicator must wait until crosslinking has been completed to apply another coat and the bond of that recoat will be wholly or partially mechanical rather than chemical.
The current pre-cats are almost trouble-free when used properly. Most are supplied ready to spray and are almost as forgiving and versatile as nitrocellulose and acrylic lacquers that they replace. They now have good shelf life, generally no pot life issues, flow and dry well and work well with the stains, glazes, toners, grain fillers, colorants and other additives you normally use. They can be used as a sealer, but catalyzed or uncatalyzed vinyl sealers are usually recommended for enhanced moisture resistance as required by Kitchen Cabinet Manufacturers Association standards.
They don’t require extensive changing of application equipment or practices, are repairable with readily available touchup products and are competitively priced with nitrocellulose lacquers of comparable quality. They also rub out well and come in a variety of sheens. Solids content, by volume, ranges from about 15 to 25 percent.
Todays’ premium pre-cats provide a more durable finish, offer more chemical and moisture resistance, and are almost as easy to use as straight lacquers. They also fill the gap between those and the finicky post-catalyzed products. But is that enough reason to change? After all, what you’ve been using for years is presumably working out pretty well.
Even though these products are catalyzed at a much lower level than the post-cats, they do contain a catalyst and over time the chemical reaction (crosslinking) can occur in the can. How long the product will continue to perform as expected depends somewhat on the storage temperature (heat accelerates the chemical reaction). The partially crosslinked liquid might still spray, flow and dry well, but might not fully develop the properties desired. Adhesion between coats could be compromised and, without destructive testing, this can go unobserved. There is the increased possibility of cracking of the film with material used past its performance life.
Manufacturers have employed various methods of addressing this issue, including making shelf-life limitations explicit in the product data sheets. Other options are to have the distributor add catalyst immediately prior to delivery or purchase smaller quantities.
Some pre-cats offer the option of adding additional catalyst to enhance the performance properties further. However, this can lower the pot life to as little as one day. There’s also the risk of adding too much catalyst, which will make the coating brittle.
Pre-cats can lift if they cure before recoating, so the instructions regarding recoat intervals should be followed. You can also run into problems by using too much thinner, strong solvents and retarders. For example, using a retarder during a repair process can cause wrinkling of the film; however since the pre-cats have better resistance to water stains there might be less need for those products.
Because the catalyst is an acid, stainless steel or plastic is recommended for spray guns and cups. One manufacturer goes so far as to recommend against the use of steel wool between coats.
It’s interesting to note that the large furniture manufacturers are still using straight lacquers. But cabinet manufacturers, refinishers and small- to mid-sized shops have been more willing to make the switch, making pre-cats a top seller. And, for what it’s worth, a high-quality pre-cat is my lacquer of choice. I encourage you to give it a try.
This article originally appeared in the April 2013 issue.