It’s winter and it’s cold. All finishes dry and harden slower in cold temperatures. Some, such as conversion varnish, don’t cure properly at all.
Slower drying increases the likelihood of sagging on vertical surfaces, dust nibs drying in the finish, and press marks when stacking.
It’s difficult keeping a shop at around 70 degrees in the winter when exhausting warm air along with the overspray through a spray booth. Generating enough warm replacement air requires expensive heating equipment and raises utility costs considerably.
It wasn’t by anyone’s design (so far as I can determine), but we now have a very interesting partial solution to the problem of getting finishes to dry quickly in a cold shop. The new emphasis on lowering the VOC content of lacquers has led to the development of a lacquer formulation that dries quickly in cold temperatures. But few people seem to realize it.
I’ve written about the solvents in lacquer several times in this column, most recently in the September 2008 issue (“Understanding solvents is a family affair,” available at www.woodshopnews.com).
Sixty-five to 75 percent of lacquer and pre-catalyzed lacquer is solvent. Six or more solvents are used, each evaporating at a different rate. Most or all of these solvents are VOCs (volatile organic compounds) that are classified as pollutants.
The current rules issued in some parts of the country, notably California, concerning the solvent content in lacquers limit VOCs to 27.5 percent. This is stated as 275 grams per liter (275 g/l).
With such a big gap between the allowed 27.5 percent and the normal 65 to 75 percent, how do manufacturers make lacquer that complies with the rule? They do this by making up the difference with acetone, which is not counted as a VOC.
I can’t give you any logical reason why acetone should be considered OK for the environment when all the other ketones are not. But I’m not complaining because the “delisting” of acetone makes the manufacture of lacquer still possible under the strict rules.
Acetone and lacquer
Acetone is an active solvent for nitrocellulose, CAB-acrylic, pre-cat and post-cat lacquers. You or the manufacturer can add all the acetone you want to any of these lacquers without causing any curing problems — except one. The finish dries very quickly because acetone evaporates rapidly.
Finishers using 275 VOC lacquer in warm climates, such as in Southern California, have found that the finish dries too quickly. It’s often not possible to spray without getting dry spray, a dull, rough surface caused by the atomized lacquer mist drying before it hits the wood.
To compensate, finishers retard the drying by adding butyl cellosolve, the slowest evaporating, widely available, active solvent for lacquer. Butyl cellosolve is not a compliant solvent, so finishers adding it are taking the finish above 275 g/l and thus breaking the law.
But it works, so many finishers do it anyway. It’s common in Southern California to see cans of butyl cellosolve in paint stores stacked right next to the 275 VOC lacquers.
How about spraying these lacquers in cold temperatures? The problem with 275 VOC lacquers is that they dry much faster than regular lacquer, often too fast. But faster drying is exactly what you want in cold temperatures to make the lacquer dry at a “normal” rate.
Lacquer is an evaporative finish. That is, it turns from a liquid to a solid entirely by the evaporation of its solvents. Even pre-cat and post-cat, which include some crosslinking, dry primarily by evaporation.
So if the drying slows in cold temperatures because solvents evaporate slower when it’s colder, then lacquers loaded up with very fast-evaporating acetone should dry faster — and they do.
The new 275 VOC compliant lacquers, which cause dry-spray problems when used in warm temperatures, can be used effectively to speed drying in cold conditions.
Using 275 VOC lacquers
Just as with all lacquers, you can spray 275 VOC lacquers without thinning if you use enough air pressure to atomize properly. It’s my experience, however, that most finishers thin their lacquer somewhat, even up to half in some cases, with lacquer thinner.
You can do the same with 275 VOC lacquers, of course. But if your intention is to compensate for cold temperatures, thinning with regular lacquer thinner will largely negate the benefit you’ve gained from using this finish. Regular lacquer thinner will slow the drying considerably because the solvents in this thinner evaporate much slower than acetone.
Instead, you should thin with acetone. You can add as much as you want and, as an added benefit, if you live in a restricted area, not take the lacquer out of compliance.
If you’re not in a restricted area, however, 275 VOC lacquers may not be available from your local supplier. But most manufacturers make this finish, so get your supplier to stock it, or you can order it online and have it shipped directly to you.
Using 275 VOC lacquer, thinned with more acetone if necessary, will create drying that is close to normal in temperatures as low as 50 degrees. Of course, you can use this lacquer in even colder conditions, but the drying will be slower the colder it gets.
If you can’t easily get 275 VOC lacquer, an alternative is to thin regular lacquer with acetone or a “fast” lacquer thinner from an auto-body supply store.
Acetone will provide the fastest drying because it is the fastest evaporating, commonly available, lacquer solvent. Fast lacquer thinners are blends of solvents, usually with a recommended temperature range listed on the can for achieving normal drying.
Adding acetone or fast lacquer thinner doesn’t make the regular lacquer equivalent to 275 VOC lacquer in drying speed because of the slower evaporating solvents already in the lacquer. But the added thinner evaporates very quickly, leaving a thinner coat that dries noticeably faster.
Fast lacquer thinners have been available to auto-body refinishers working in cold shops for as long as I’ve been doing finishing, though these solvents are becoming harder to find because the body guys are getting away from using lacquers. I’ve never understood why our wood-finish suppliers haven’t offered this solvent to us.
Think of a fast lacquer thinner as the opposite of lacquer retarder. You add retarder to slow the drying of the finish in humid or very hot conditions. You add fast lacquer thinner (or acetone) to speed the drying in cold conditions.
As I’ve pointed out many times in this column, the primary advantage of finishes that thin with lacquer thinner is the control we have over the drying speed in differing weather conditions.
Now, in a serendipitous stroke of luck, we have available a fully functional lacquer, not just thinners, we can use to overcome slow drying in cold conditions.
Bob Flexner is author of “Understanding Wood Finishing.”
This article originally appeared in the March 2009 issue.