Michael Dresdner

straight talk about wood finishing

Q: You make a distinction between lacquer and oil based finishes. Why does it matter?
A: Lacquer, which is a category that includes shellac, is an evaporative finish. It cures solely by the solvent evaporating. That means the molecules floating around in the solvent when it is in its liquid form are exactly the same once the solvent evaporates. The film is formed by these long, gangly molecules twisting and clumping together, but not bonding chemically. Technically, they form an amorphous solid.
Because of that, evaporative finishes have two very distinct characteristics. First, they will redissolve with their own original solvent; lacquer with lacquer thinner, shellac in alcohol. Second, when subjected even to fairly low levels of heat, they start to liquefy. Hence, they are known as having poor solvent and heat resistance.
Oil based finishes cure very differently. They actually go through a chemical change in which small molecules link up and form larger molecules, eventually forming a solid. Though most do, an oil based finish does not need to have solvent in it to work. A good example of a solvent free finish is pure boiled linseed oil. Although no solvent evaporates, it goes from liquid to solid, using oxygen absorbed from the air to bond its molecules chemically. Epoxy, conversion varnish, polyester, UV coatings and even some types of polyurethane all bond through chemical reaction rather than solvent evaporation alone. Such finishes will not dissolve in their original solvent, and typically are thought of as having much higher chemical and heat resistance.

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