Understanding Solvents: Part III (Laquer Thinner)

Lacquer thinner is the solvent and thinner used with all types of lacquer, including nitrocellulose, CAB-acrylic and catalyzed. It’s the most interesting of the finish solvents because it’s composed of half-a-dozen or so individual solvents. Manufacturers vary these to control solvent strength and evaporation rate.

Solvents from all five of the solvent families are used in lacquer thinners. (See Understanding Solvents, Part II). Toluene, xylene and “high-flash” (meaning fast evaporating) naphtha are from the petroleum-distillate family. The other four families are ketones, esters, glycol ethers and alcohols.

All the individual solvents from the ketone, ester and glycol ether families dissolve lacquer on their own and are called “active” solvents. But they evaporate at different rates, so manufacturers choose among them to make a thinner that evaporates in steps at the speeds they want.

Alcohol doesn’t dissolve lacquer on its own, but it does in combination with these other solvents, so it is called a “latent” solvent. One or more of the alcohols is usually added to the mix to reduce cost.

The nature of lacquers is that they can be fully dissolved (meaning the lacquer molecules are separated) and still be too thick to spray without getting severe orange peel. So to further “thin” the lacquer (and also lacquer thinner) without adding expensive dissolving solvents, manufacturers add up to 50% toluene, xylene or high-flash naphtha.

These solvents are called “diluents” or “diluting” solvents.

Because the diluting solvents don’t dissolve lacquer, they have to evaporate fast enough to be out of the lacquer before all the dissolving solvents have evaporated. Otherwise, the lacquer will come out of solution and appear as white, cotton-like, particles on the dried finish.

Solvent Strength

By varying the individual solvents and the ratios used, manufacturers control the strength of the lacquer thinner and the speed the lacquer dries.

Strength is important to insure the lacquer fully dissolves. All commonly available lacquer thinners sold for thinning lacquer will adequately dissolve the lacquer. But less expensive lacquer thinners sold for clean up don’t dissolve lacquer adequately. If too much of this thinner is added, the lacquer will come out of solution.

Automotive lacquers require a higher ratio of dissolving solvents than do wood lacquers. So thinners sold for automotive lacquers are more expensive, but they still work well with wood lacquers. On the other hand, lacquer thinners sold for wood lacquers may not be strong enough to thin automotive lacquers.

Multiple Solvents

Arguably, the most unique characteristic of lacquer finishes, and the reason so many finishers love spraying lacquer, is its resistance to running and sagging of vertical surfaces. The finish can be sprayed quite thick without problems. It’s hard to screw up a sprayed lacquer finish.

The explanation is the lacquer thinner. It is composed of a number of individual solvents that evaporate at different rates.

Lacquer is a thick finish that requires a lot of solvent (as much as 75% or more) to make it thin enough to get through the nozzle on the spray gun without orange peel. But once the finish gets through nozzle, it no longer needs to be so thin. It no longer needs so much thinner.

So individual solvents are chosen to evaporate very quickly after the lacquer finish is sprayed. These solvents, beginning with those that don’t dissolve the lacquer, evaporate one after another, beginning as soon as the finish leaves the spray gun, so the lacquer thickens quickly on the surface. Some slower evaporating solvents, called “tail” solvents, remain for a while to allow the lacquer to level out.

To better understand what’s happening, please refer to the Table of Solvents, which I include here only for illustration, not with the idea that you should have to learn these names. These are the most common individual dissolving solvents added to lacquer and used to make up lacquer thinners.

The solvent, butyl acetate, almost halfway down in bold, is used as the standard to which the other solvents are compared. It is assigned the value of “1.”

So acetone, at the top of the list with a value of 5.7, evaporates 5.7 times faster than butyl acetate, and Butyl Cellosolve, at the bottom of the list with a value of .08, evaporates about 1/12 as fast as butyl acetate. Acetone evaporates very rapidly; Butyl Cellosolve evaporates very slowly; and all the other solvents listed evaporate somewhere in between.

Include three or four of these solvents together with some very fast evaporating toluene or high-flash naphtha and it’s easy to understand how the sequential evaporation of each solvent causes lacquer finishes to seize up quickly on vertical surfaces so they don’t run or sag.

Fast and Slow

It’s also easy to understand how lacquer thinners can be made to evaporate faster or slower simply by choosing solvents nearer the top of the list or nearer the bottom of the list.

Lacquer retarders, used to eliminate blushing (turning white) on humid days and eliminate dry spray (a sandy surface) on hot, dry days or when spraying the insides of cabinets, are made up of individual solvents nearer the bottom of the list.

The slowest evaporating retarder and the most effective in extremely humid conditions, such as those found near the Gulf of Mexico, is Butyl Cellosolve, which is commonly sold separately as a “super” retarder.

Be aware that adding any retarder to lacquer slows the drying and may affect your production (more time between coats and more time before you can deliver or stack parts).

Fast lacquer thinners, usually available from auto-body supply stores but not from wood-finish suppliers, are made up of solvents nearer the top of the list. These thinners make it possible to spray with near normal drying times in cold temperatures.

Acetone can also be used to speed the drying of lacquer in cold temperatures. You add it to the finish similar to the way you add retarder, judging how much will be necessary to achieve the drying rate you want. It’s always trial and error with both retarder and with acetone or fast lacquer thinner.

Standard lacquer thinners from different manufacturers all dissolve and thin lacquer adequately, but they may differ somewhat in their evaporation rate. If you switch brands of lacquer thinner, you may have to adjust your finishing schedule.

Brushing Lacquer

Some lacquers are made for brushing. To achieve this, manufacturers simply use slower evaporating solvents to dissolve the lacquer. Spraying one of these brushing lacquers requires more attention because they have a much greater tendency to run and sag on vertical surfaces, and they slow production because they dry slower.

Restricted Areas

Some parts of the country have VOC laws that restrict the percentage of solvent that can be included in a finish. Typically, these laws restrict lacquer to 27.5 percent VOC solvent, 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 between 27.5% and about 75% 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 to the lacquer. It’s legal to sell and buy this solvent, but you should be aware that adding it to your lacquer may take it out of compliance.

Table 1 – Solvent Evaporation Rates