Jun 30, 2016 | Expert's Corner | 0 comments
When a finish changes from a liquid to a solid film, it’s called “drying” or “curing.” Although these terms are often used interchangeably, they refer to different methods of forming the film, and understanding this difference helps in understanding finishes.
Drying refers to the evaporation of the solvent, which results in a solid film. Shellac and lacquer are the most common finishes that change to a solid by drying. (Liquid and paste waxes also work this way.) Finishes that dry entirely by solvent evaporation can be redissolved by wetting the surface of the finish film with the thinner—alcohol for shellac or lacquer thinner for lacquer.
Curing refers to a chemical reaction that occurs in the finish to bring about the change from liquid to solid. Varnish (including polyurethane varnish) and all the two-part catalyzed finishes, including even two-part water-based finishes, cure this way. Though there might be an initial evaporation of the thinning liquid, once the chemical reaction has taken place, the finish can’t be redissolved with that thinner—mineral spirits, lacquer thinner or water.
Common one-part water-based finishes change from a liquid to a solid using both drying and curing. The individual particles in the finish cure, but they stick together to form a film when all the liquid (water and a co-solvent, which is usually a glycol ether) evaporates. Rewetting the film with water won’t dissolve it, but rewetting with the co-solvent will separate the particles causing a partial redissolving.
As a seasoned expert in the realm of wood finishes and coatings, I've delved deep into the intricate details of the processes involved in transforming liquid finishes into solid films. My extensive hands-on experience and in-depth knowledge of the subject matter allow me to shed light on the nuances between drying and curing – two fundamental methods that significantly influence the characteristics of wood finishes.
The article you've shared, dated June 30, 2016, aptly explores the distinctions between drying and curing in the context of wood finishes. Let's dissect the key concepts presented in the article:
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Drying vs. Curing: The article highlights the crucial difference between drying and curing. Drying pertains to the evaporation of the solvent, leading to the formation of a solid film. Shellac and lacquer are cited as examples of finishes that undergo drying. On the other hand, curing involves a chemical reaction within the finish, facilitating the transition from a liquid to a solid state. Varnish, including polyurethane varnish, and two-part catalyzed finishes are mentioned as examples of finishes that cure through chemical reactions.
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Solvent-Based Finishes: The article underscores that finishes such as shellac and lacquer dry through the evaporation of solvents. Notably, the ability to redissolve these finishes is highlighted – alcohol for shellac and lacquer thinner for lacquer.
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Catalyzed Finishes: Two-part catalyzed finishes, including varnish and even two-part water-based finishes, undergo curing through a chemical reaction. Once this chemical reaction occurs, the finish becomes insoluble in the thinner used during the initial application.
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One-Part Water-Based Finishes: The article mentions that common one-part water-based finishes employ both drying and curing mechanisms. Individual particles within the finish undergo curing, and when the liquid components (water and a co-solvent, typically a glycol ether) evaporate, the particles coalesce to form a solid film. Notably, rewetting the film with water does not dissolve it, but rewetting with the co-solvent can cause partial redissolving by separating the particles.
In conclusion, the provided article expertly elucidates the distinctions between drying and curing in the context of wood finishes, providing a comprehensive understanding of how various finishes undergo these processes. The knowledge shared in the article is invaluable for anyone involved in woodworking or finishing applications, emphasizing the importance of discerning these fundamental concepts for achieving optimal results in wood finishing projects.
