Popular Woodworking 2007-12 № 166, страница 61

Flexner on Finishing

BY BOB FLEXNER

A Primer on Solvents

Categorize by type to cut through confusion.

^_jnless 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,

90 ■ Popular Woodworking December 2007

Categorize. The easy way to make sense of solvents is to divide them into petroleum distillates (left) and all the others (right). Once you have understood the petroleum distillates, all of which do essentially the same thing at different evaporation rates, it's easy to handle the rest.

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