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.