|Understanding solvents is family affair|
|Solvents for water-based finishes|
Though it’s not critical for our understanding of solvents, notice that these petroleum distillates become oilier as they slow in evaporation rate. Mineral spirits is oilier than naphtha, and kerosene is oilier than mineral spirits.
The same phenomenon occurs in each solvent family. As you move down the list of solvents from fastest evaporating to slowest evaporating, the solvents become oilier.
Lacquer thinner is different from other solvents because it is made up of a number of individual solvents, often six or more.
The solvent families that dissolve nitrocellulose and CAB-acrylic lacquers, and are used in catalyzed lacquers, are the ketones, esters and glycol ethers. These are the “active” solvents used in lacquer thinner. Manufacturers choose between dozens of individual members for cost and evaporation rate.
Alcohols, which dissolve and thin shellac by themselves, also have some dissolving power on lacquer when combined with one or more of the active solvents. Alcohols are usually added to lacquer thinner to reduce cost.
The cost can be reduced even more because it doesn’t take much solvent to dissolve lacquer. It just takes a lot to thin it enough to spray. So to make lacquer thinner cheaper to produce, manufacturers add 40 to 50 percent petroleum distillates to “thin” the lacquer thinner.
The active solvents dissolve the lacquer, and the petroleum distillates separate the long stringy lacquer molecules enough so they aren’t bumping into each other. This allows the finish to be atomized well during spraying to reduce orange peel.
Not just any petroleum distillate can be added to lacquer thinner. It’s critical that the petroleum distillate evaporates before the active solvents or the lacquer will come out of solution and result in cotton blush, which shows up as opaque white clumps of finish on your work. The petroleum distillates used in lacquer thinner are toluene, xylene and high-flash naphtha, all of which evaporate very rapidly.
Two critical advantages are gained by using a number of solvents that evaporate at different rates.
The first is reduced sagging on vertical surfaces. Once the lacquer has exited the spray gun, a large amount of thinner isn’t needed anymore. So some of the faster solvents, including the petroleum distillates, evaporate even before the lacquer hits the substrate or very quickly thereafter.
The remaining solvents continue evaporating quickly, one after another, leaving only a little solvent in the finish so it can level out. As a result, the lacquer thickens quickly.
Unlike spraying water-based finishes or brushing varnish, you have to spray lacquer really thick in one place to create a run on a vertical surface.
The other advantage is that lacquer thinner can be made to evaporate faster or slower simply by including solvents that evaporate faster or slower. So lacquer thinners of various evaporation rates can be used to control drying in all sorts of weather conditions: hot or cold, damp or dry.
Use slower-evaporating lacquer thinner (“retarder”) in damp weather to eliminate blushing and in very hot, dry conditions, or on the insides of cabinets, to eliminate dry spray. Use faster evaporating lacquer thinner, available at auto-body supply stores, in cold conditions to make the finish dry at a normal rate.