One aspect of finishing that doesn't get much attention is the vast array of solvents used in the process. Having the right solvent on hand is often more an afterthought than a primary concern. That is until you want to thin a finish or clean a brush and you don't have the solvent you need or aren't even sure what that might be. The upshot is that it pays to have a basic knowledge and understanding of the properties and uses of the different finishing solvents.
WHAT DO THEY DO? All finishes contain one or more solvents. They serve two purposes with a fine distinction. Solvents are primarily used to dissolve the resins in the finish so they can be applied to the surface of the wood.
In other instances, solvents are simply added to the finish to reduce the viscosity or "thin" the finish, making it easier to apply. So what we lump together as solvents can either be a true solvent or simply a thinner. Sometimes a solvent can serve both purposes. Here, we'll just consider them all as solvents.
MILD TO STRONG. Solvents can be generally classified in terms of their ability to dissolve different finishes and their volatility level (evaporation rate). I simply think of solvents as falling in a range from mild to strong. This is an "unscientific" distinction, but it provides a good starting point.
TURPENTINE. At one time, turpentine was the solvent used in all paints, varnishes, and other finishes that used linseed oil as a base. It's made by the distillation of gum or sap from certain pine trees. And its strong, "piney" odor is pretty unmistakable.
Today, turpentine has limited use in finishing. Its slow evaporation rate has the undesirable effect of extending the drying time of already slow drying varnishes, paints, or oil finishes. But it's still used in some traditional finishes such as beeswax and turpentine.
MINERAL SPIRITS. Mineral spirits have replaced turpentine as the solvent and thinner for varnishes, other oil-based finishes, and oilbased paints. Mineral spirits are distilled from petroleum and are less labor intensive and costly to manufacture than turpentine.
But not all mineral spirits are equal which leads to confusion. Depending upon minor differences in the formulation, mineral spirits can be labeled odorless, low odor, or standard mineral spirits. Besides strength of odor, each type has slightly different solvent properties. The odorless and low-odor types tend to evaporate faster and have a bit weaker solvent effect.
Any of the three will work about equally well with finishes requiring mineral spirits. The low-odor types can be more expensive, but if you're sensitive to the fumes, it might be worth the extra cost.
PAINT THINNER? Is there a difference between mineral spirits and paint thinner? The answer is maybe. Often, products labeled as paint thinner are pure mineral spirits. When this is the case, it's stated on the can. In other instances, paint thinners may contain various cheaper solvents. When thinning finishes, it's probably a good idea to stick with pure mineral spirits.
NAPHTHA. I consider naphtha sort of a "second string" solvent with a few specific uses. Like mineral spirits, it's a petroleum distillate. It has a fairly low solvent strength but a fast evaporation rate. This is what makes it useful.
Naphtha can be used as a thinner if a faster drying time is desired, such as when applying paste wood filler or oil-based stain. It can also be used as a lubricant and cleaner when rubbing out a finish.
DENATURED ALCOHOL. Shellac is my favorite all-purpose sealer. And consequently, I always keep denatured alcohol on hand. It serves as both the solvent for the shellac resin and as the thinner used to dilute it for application.
Denatured alcohol is simply ethanol that has poisonous methanol added to it (or denatured) to prevent consumption. It's highly volatile. And this is what makes shellac such a handy sealer - it dries in minutes. Denatured alcohol is also used as the solvent for certain NGR (non grain-raising) dyes.
LACQUER THINNER. Lacquer thinner combines high volatility with high solvent strength. This is why it's such a useful finishing solvent. As well as being the thinner for nitrocellulose lacquer, it will soften or redissolve many other types of finishes and makes a good allpurpose cleaner. But the fumes are very strong and can be harmful.
Lacquer thinner is actually a mixture of chemicals. This feature means lacquer thinner can be custom blended for different uses such as thinning a slower-drying brushing lacquer.
THE REST. You'll find a number of other solvents on the store shelf that fall into the "highly volatile" category - acetone, methyl ethyl ketone, and xylol for example. Some of these are the components of lacquer thinner. They are mainly used as thinners for specialty finishes and also make effective cleaners, but generally have little home shop use.
SOLVENT SAFETY. When working with solvents, it pays to take some simple precautions. First, don't overlook the hazard of fire. Be sure to store the solvents appropriately. And second, during any prolonged exposure, I always wear proper protection (photo below).
Knowing your solvents isn't rocket science - just a little chemistry. But when the goal is a great finish, it's worth the effort.