Understanding Lacquer Thinner

Three thinners. You have far more control of the drying rate of lacquer-type finishes than other finishes. To speed up the drying, add a little acetone. To slow the drying, add some lacquer retarder. Because of the variables you can’t know how much to add except by experimentation, but you have a wide range available to you.

This solvent is unique.

Lacquer thinner is the thinner used for all types of lacquer (not water-based finishes, which are sometimes misrepresented as “lacquer”). These include the most common lacquer –nitrocellulose lacquer, colorless CAB-acrylic lacquer and the most durable lacquer – catalyzed lacquer.

Of all solvents used in wood finishing, lacquer thinner is by far the most unique because it is the only one made up of a half-dozen or so individual solvents. By varying the solvents used, manufacturers control the strength of the lacquer thinner and the speed of evaporation.

Even if you don’t use lacquer, understanding a little about lacquer thinner will help you understand the other thinners used in finishes – denatured alcohol, mineral spirits and water – and, most important, the limitations of these thinners compared to lacquer thinner.

Solvent Strength

The various types of lacquers don’t need a whole lot of solvent to put them into solution. But the nature of these finishes requires a lot of thinner to make them thin enough to brush or spray. This is because the molecules of these lacquers are very long and skinny, shaped somewhat like strands of spaghetti. It takes a lot of thinner to separate these molecules enough so they don’t bump against each other, which has the effect of making the finish thick, or viscous.

So, to lower the price of lacquer thinner, manufacturers add a significant amount of what are called “diluting” solvents. These solvents don’t actually dissolve the lacquer. But, critically, they evaporate more rapidly than the dissolving solvents so the lacquer doesn’t come out of solution.

The ratio of dissolving to diluting solvent varies depending on the intended use for the lacquer thinner. For example, automotive lacquers require more solvent strength than wood lacquers. So a higher ratio of dissolving solvent is used, making these lacquer thinners more expensive.

An automotive lacquer thinner, available from auto-body stores, will dissolve and thin wood lacquer fine, but a wood lacquer thinner may not work well in an automotive lacquer.

Similarly, a lacquer thinner meant just for cleaning spray guns doesn’t have to be as strong as a lacquer thinner meant for thinning wood lacquer. So less dissolving solvent is included in clean-up thinners to reduce their expense.

Don’t use a clean-up lacquer thinner for thinning any type of lacquer because the thinner will probably take the lacquer out of solution. The sprayed surface will be covered with small white particles resembling cotton. This is called “cotton blush.”

Cotton blush. If you thin lacquer with “clean-up” lacquer thinner, the lacquer will probably come out of solution and show up on the surface as small cotton-like particles. Always use a lacquer thinner meant for thinning lacquer.

In case you’re interested, the dissolving solvents are ketones, esters and glycol ethers. Alcohols also have a dissolving effect when combined with the other dissolving solvents. The diluting solvents come from the hydrocarbon family and include toluene and high-flash (fast evaporating) naphtha.

Evaporation Rate

Manufacturers can make lacquer thinners that evaporate faster or slower simply by which individual solvents they choose to include.

There’s no reason, except expense, that a lacquer thinner couldn’t be made with just one dissolving solvent if it evaporates at a proper rate. But there is a really big advantage to using a number of solvents that evaporate at different rates: resistance to sagging when the finish is sprayed onto a vertical surface.

If you have ever sprayed a finish that thins with lacquer thinner and compared the experience to spraying any other finish – for example, polyurethane, water-based finish or shellac – you must have noticed how easy it was to spray the lacquer-type finish without runs and sags. And how difficult it was to do the same with the other finishes.

This is because the solvents in lacquer thinner are chosen specifically to evaporate at different rates so the lacquer thickens quickly. Some of the solvents, especially the diluting solvents, begin evaporating even before the sprayed finish reaches the target. Others, including some of the dissolving solvents, remain in the finish for five or 10 minutes to allow it to level out.

This step-by-step drying, made possible by the varying evaporation rates of components of the thinner, is the principle quality of lacquer that makes it loved by professional finishers. It’s so easy to spray a flawless finish.

Problem Solving

With dozens of individual dissolving solvents available, it should be obvious that brands of lacquer thinner can vary noticeably in evaporation rate. You should be aware of this possibility if you switch brands. You may have to adjust your timing.

It should also be obvious that lacquer thinners can be made to evaporate very slowly or very rapidly to make possible lacquers that can be brushed, called “brushing lacquer,” and lacquers that can be sprayed in extreme weather conditions.

Dry spray. Spraying in hot temperatures, or spraying the insides of cabinets and drawers, can lead to dry spray that creates a sandy look and feel. The lacquer is drying so fast that it turns to dust before it lands on the surface. To avoid dry spray, add some lacquer retarder to the lacquer.

For example, lacquer “retarders” are made with slower evaporating solvents. You can add a little retarder to your lacquer in hot, dry conditions to avoid “dry spray,” which is a sandy surface caused by the lacquer drying so fast that it has turned to dust before it reaches the target.

Retarders also make possible the avoidance of “moisture blushing” in humid conditions. Condensed moisture enters the finish, which then dries before the moisture can evaporate. When the moisture does evaporate, air voids are left which cause the finish to appear milky white. Slowing the drying allows time for the moisture to evaporate before the finish dries so completely.

Moisture blush. A very common problem when spraying lacquer is moisture blush, which occurs in humid conditions, and appears milky or cloudy. To avoid this problem, add some lacquer retarder. It will slow the drying of the lacquer, allowing time for the moisture to evaporate.

At the opposite end, some automotive lacquer manufacturers supply “fast” lacquer thinners that make the lacquer dry at a normal rate in cold temperatures, even as low as 45° F or 50° F. These thinners are available at auto-body supply stores. Adding acetone will achieve the same end, and it is widely available.

Restricted Areas

Some parts of the country have volatile organic compound (VOC) laws that restrict the percentage of polluting solvent that can be included in a finish. Typically, these laws restrict lacquer to 27.5 percent, which is way too little for spraying.

Acetone, however, is an exempt solvent. It can be added to lacquer in any amount, so manufacturers typically make up the difference with acetone.

This has two impacts. First, it makes the lacquer more expensive. Second, and much more significant, it makes the lacquer dry so fast it can’t be sprayed in warm temperatures without getting dry spray. (The lacquer works great in cold temperatures, however.)

Finishers get around the fast drying by adding butyl cellosolve, the slowest evaporating retarder, to the lacquer. It’s legal to sell and buy this solvent everywhere, but you should be aware that adding it to your lacquer may take it out of compliance with local laws.

Lacquer thinner is indeed a unique solvent, especially when contrasted with the other solvents used for finishes.