Working Around the Weather
Most cans of finish list optimal drying times as if we all lived in San Diego: a temperature of 70°F and 70% relative humidity. (See label at right.) But for the rest of us—in the arid Southwest, the muggy Southeast, the temperature-extreme Midwest or East Coast—how can we ensure that our finishes dry quickly, smooth, and dust-free?
Finishing in a cold shop causes brushed-on finishes to flow out slowly. And on a hot, dry summer day, finishes dry so fast that brush marks become a problem. Knowing how to work within the weather conditions, or alter them, gives better results. Here's how.
No uncertain terms
Although the terms "solvent" and "thinner" are often used interchangeably by woodworkers, they represent two different concepts. Solvents dissolve something; thinners just spread it around. For example, mineral spirits dissolves wax, so for wax, it's a solvent. However, mineral spirits doesn't dissolve varnish, it simply spreads the varnish molecules farther apart, making them easier to brush onto wood. So for varnish, mineral spirits acts as a thinner.
A brief lesson in how finishes dry
Finishes dry by three different methods, and the more complex the method, the more finicky the finish when it comes to the weather. From least to most complex:
■ Evaporative finishes, such as shellac and lacquer, dry simply when the solvent evaporates, leaving a hardened finish film on the wood surface. Weather conditions affect their dry times the least.
■ Reactive finishes, such as oil-based varnish, dry in two steps: First, the thinner evaporates and the finish gets tacky. Then, a chemical reaction (polymerization) between the finish resins and oxygen in the air occurs, causing a hardened film to develop. In colder than ideal conditions, the reaction slows, delaying hardening.
■ Coalescing (water-based) finishes dry in three steps. First, the water (the thinner) evaporates quickly; then, the slower-evaporating solvent, which has softened the droplets of finish floating in it, evaporates. Finally, the droplets left behind join together and harden—or coalesce—into a film. Because of the delicate relationship between the thinner and solvent evaporation times, temperature and humidity levels greatly impact drying times. As you can see in the chart above right, water-based finishes prove the most difficult to apply at low temperatures, so follow the finish manufacturer's application recommendations as closely as possible. And know that, due to differences in finish formulas, those specs may vary widely from manufacturer to manufacturer or even among finishes from the same maker (brush-on poly vs. aerosol poly, for example).
The common denominator in all three types of finishes: The solvent or thinner must evaporate before the finish can dry. And this happens best with warm air and low humidity.
Cold air: Freshen it with motion
One way to help finishes dry faster in cold conditions is to create air movement. That's because on a hot day, warm air naturally rises, drawing in cool air to replace it at the surface of your project. But on a cold day, the air stagnates, so less air passes over the project, which slows drying.
By establishing some airflow, even slightly, the air at the project surface remains fresh and able to absorb more thinner or solvent. This also ensures a fresh and constantly renewing source of oxygen to maintain the chemical reaction that occurs in reactive finishes. Get a steady air path going through your shop, but do it without aiming fans directly at your project, as shown in the drawing above.
Don't complain about the weather; do something
Take charge of the weather by altering your finish or your choice of finish. For example, in hot weather, you can thin a finish to allow greater time for flow-out and prevent brush marks. But remember that thinned finish means more coats to build up the film, and longer dry time means the possibility of more dust on the wet and tacky surface.
To really speed up drying in cold months (think: it's Christmas Eve, and the projects have to be dry, wrapped, and under the tree the next morning), choose a different finish altogether. Shellac, a wiping varnish, and spray lacquer dry quicker than other finishes in cold weather.
The best solution, though: Wait for a day that's not too cold, too hot, or too humid. And then make sure the wood, the finish, and the air are all the same temperature before you start brushing or spraying.