A Primer on Solvents

Unless you’ve been doing wood finishing for quite a while, I imagine the shelves of solvents in home centers and paint stores look pretty much like Greek to you. Which solvent goes with what? How does one make sense of all the possibilities?

Here’s the easy way to understand solvents for wood finishes.

First, divide the solvents between the petroleum distillates, including turpentine, and all the rest. Because most of the solvents on the shelves are petroleum distillates, this reduces the remaining products to a number that’s easy to handle.

Then make sense of the petroleum distillates and turpentine, all of which do essentially the same thing at different evaporation rates, and when this is done, deal with what is left.

Petroleum Distillates
Petroleum distillates are all distillations of petroleum. They include mineral spirits (paint thinner), naphtha, toluene, xylene and some “turpentine substitutes” such as Turpatine and T.R.P.S. The primary use for these solvents in wood finishing is thinning waxes, oils and varnishes, including polyurethane varnish, and cleaning brushes. The solvents are also used to clean oil, grease and wax.

Turpentine is a distillation of pine-tree sap. Before the mid-20th century, turpentine was widely used as a thinner and clean-up solvent for oil paint and varnish, and also as a grease and wax cleaner.

With the growth of the automobile industry and its need for petroleum products, a large number of petroleum solvents were introduced and these have almost entirely replaced turpentine because they are less expensive and have a less unpleasant odor. The only sector in which turpentine is still used in any significant quantity is fine arts.
To distill petroleum, it is heated higher and higher and the gases released at different temperatures are condensed into the various liquid solvents.

The first gas to come off is methane, which doesn’t condense at room temperature, only at much colder temperatures. Then there’s ethane, propane, butane, etc. Heptane and octane are used to make gasoline, a liquid that evaporates very rapidly. Gasoline is sometimes used as a cleaner, but it is very dangerous because it is explosive. About 20 years ago the retired local sheriff in my town, an amateur woodworker, died of burns he received in an explosion while using gasoline for cleaning.

The solvents we use in wood finishing evaporate much more slowly than gasoline and are relatively safe to use, even with poor ventilation. But it’s still unwise to use them in a room with a flame such as a pilot light.

Mineral Spirits and Naphtha
The two most widely used finishing solvents are mineral spirits and naphtha. For our purposes, the principal differences between the two are evaporation rate and oiliness. Naphtha evaporates more quickly than mineral spirits and is “drier,” that is, less oily. Naphtha is therefore better for cleaning all types of oily, greasy or waxy surfaces. Mineral spirits is better for thinning oils, varnishes (including polyurethane varnish) and oil-based paints because it leaves more time for the coating to level after brushing.

Naphtha is a stronger solvent than mineral spirits, but this is rarely significant in wood finishing. Mineral spirits is strong enough for any normal operation.

To better place turpentine among the petroleum distillates, think of it as having the solvent strength of naphtha but the evaporation rate and oiliness of mineral spirits. I don’t know of any situation in wood finishing where this is important.

The nickname for mineral spirits is “paint thinner.” Back in the early days of mineral spirits, before World War II, all paints were oil-based. So there was only one thinner for paint. The nickname made sense.

Today, with water-based paints and finishes in wide use, the name could be confusing to beginners. Paint thinner is used only with oil-based paints and finishes.

It’s important to emphasize that mineral spirits and paint thinner are the same thing. Amazingly, there are manufacturers who try to trick you into paying more by labeling their containers “pure” mineral spirits and charging more.

The common naphtha available in paint stores is VM&P Naphtha. VM&P stands for “varnish makers and painters.” Stronger and faster evaporating naphthas exist, but these are rarely sold to the general public.

Toluene and Xylene
Toluene, nicknamed “toluol,” and xylene, nicknamed “xylol,” are the strong, smelly, fast evaporating and “dry” parts of mineral spirits and naphtha. These solvents are removed from mineral spirits and naphtha at refineries and sold separately as cleaners, and also as solvents for some high-performance spray finishes such as conversion varnish. Toluene and xylene are very effective as cleaners, but I find naphtha adequate for almost all situations.

Toluene evaporates a little more quickly than xylene, but this is significant only when using the solvent as a thinner.
The problem with these two solvents is that they are relatively toxic. They will affect your nervous system causing irritability and drunkenness, and in large doses could cause serious health problems. You should never use them in any sizeable quantity in a room without good exhaust.

One very interesting use for toluene and xylene is to soften latex paint. Using a dampened cloth (and solvent-resistant gloves) you can easily remove latex paint that has spattered off a paint roller, or even a full coat of latex paint, from any finish except water-based finish, without causing any damage to the underlying finish. In fact, the products sold specifically to do this, “Oops!” and “Goof-Off,” are principally xylene.

Because white and yellow glues are the same chemistry as latex paint, you can also use toluene or xylene to soften and scrub these glues from wood when you have glue seepage or fingerprints that you didn’t fully remove during sanding. You will need to use a toothbrush or soft brass wire brush to get the glue out of the pores.

Odorless Mineral Spirits
The mineral spirits left after the toluene and xylene are removed is sold as “odorless” mineral spirits. When understood this way, it’s obvious that odorless mineral spirits is a weaker solvent than regular mineral spirits. But I’ve never found this to be a problem. It still appears to be strong enough to thin all common oils, varnishes and oil paints.

The disadvantage of odorless mineral spirits, of course, is that it is considerably more expensive because of the extra steps necessary to produce it. You may find the extra expense worth it, however, just to avoid the unpleasant odor of regular mineral spirits.

Turpentine Substitutes
The so-called turpentine substitutes are an interesting breed. My first question when I talk to the companies that produce them is, “Isn’t that the role of mineral spirits?” (One company spokesman, identified as the “chemist,” explained that these products were necessary because of all the protests against cutting down trees to make turpentine! Of course, trees aren’t cut down; the sap is drained.)

Actually, these solvents seem to have similar characteristics to turpentine in that they have the solvent strength of naphtha but an evaporation rate closer to mineral spirits. So they are useful to fine artists but provide no special benefit to wood finishers.

These are all of the petroleum distillates used in wood finishing. Now for the other solvents.

Alcohol is the solvent for shellac. The solvent dissolves solid shellac flakes and thins the liquid shellac after dissolving. There are two alcohol types available at paint stores: methanol and denatured.

Methanol evaporates a little faster than denatured, but it is toxic and could blind or even kill you if you breath too high a vapor concentration for too long. You shouldn’t use it unless you have good ventilation in your shop.

Denatured alcohol is ethyl alcohol (the alcohol in beer, wine and liquor) that has been made poisonous so we don’t have to pay liquor taxes to buy it. This is the alcohol you should use with shellac.

In situations where shellac is not the finish, alcohol has the further use as a felt-pen-ink remover. Dampen a cloth and wipe over the mark and you will remove it in most cases. You won’t damage any finish except shellac as long as you don’t soak the surface.

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

Solvents from five different families are used in lacquer thinners, including toluene, xylene and “high-flash” (meaning fast evaporating) naphtha 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, 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 when in combination with these other solvents. So one or more of the alcohols is usually added to the mix to reduce cost.

The nature of lacquer is that is can be fully dissolved and still be too thick to spray efficiently. So to further thin the lacquer without adding expensive dissolving solvents, manufacturers add up to 50 percent toluene, xylene or high-flash naphtha to, in effect, “thin” the lacquer thinner.

By varying the solvents used, manufacturers can control the strength of lacquer thinner (automotive lacquers need a higher percentage of dissolving solvent) and the speed of evaporation. For example, lacquer retarders are made to evaporate slower so the lacquer stays “open” on the surface of the wood longer in order to eliminate blushing (turning white) in humid weather and dry spray (a sandy surface) in hot weather.

The purpose of using multiple individual solvents evaporating at intervals is to control the thickening of the lacquer on a vertical surface to reduce runs. The lacquer thickens quickly after being sprayed but enough of the slower evaporating solvents remain so the finish has time to flatten out. Lacquer thinner is unique among solvents for having this characteristic.

A cheaper “clean-up” lacquer thinner is often available. It’s made with a higher percentage of “thinning” petroleum-distillate solvents and doesn’t dissolve lacquer well. You will have problems if you use this thinner for thinning lacquer.

Acetone and MEK
Only one of the families of active solvents in lacquer thinner (ketones, esters and glycol ethers) is commonly available in paint stores. This is the ketone family. The two fastest evaporating ketones, acetone and methyl ethyl ketone (MEK), are usually available.

Both make excellent cleaners, but keep in mind that they will damage and remove all but the most solvent-resistant paints and finishes.

Brush Cleaners and Deglossers
Brands of brush cleaner and deglosser (liquid sandpaper) vary greatly in their composition. Some are even water-based, but these work more slowly and are less effective than solvent-based.

You can usually substitute a brush cleaner for the mineral spirits or lacquer thinner you may otherwise use to clean your varnish, lacquer or water-based finish brushes. (It’s easiest to clean shellac with household ammonia and water.) Brush cleaners are usually more expensive, however.

What is left unsaid about deglossers is that it matters greatly what paint or finish you’re trying to clean and dull. Cleaning grease or wax is no problem, but high-performance paints and finishes such as powder and UV-cured coatings, catalyzed lacquer, conversion varnish and even oil-based polyurethane are very solvent resistant. So it’s rarely possible to dull them short of abrading with real sandpaper or steel wool.

Manufacturers are very creative in their labeling, so you could easily come across solvents with different names than the ones I’m using. But if you read the intended uses listed on the containers, you should be able to place them in one of the above categories. PW