Popular Woodworking 2008-08 № 170, страница 59

Flexner on Finishing

BY BOB FLEXNER

The / Myths of Polyurethane

Oft-repeated 'rules' that are, quite simply, wrong.

A

/ Vll levels of finishing are burdened with myths, but the types of finishes used by amateurs and sold in home centers and woodworking stores suffer the most. Myths about polyurethane are a good example of the problem.

What is Polyurethane?

Oil-based polyurethane is simply a type of varnish. It's common alkyd varnish made with some polyurethane resin added. Alkyd is the resin used in almost all varnishes and oil-based paints. The polyurethane resin adds scratch, heat, solvent and water resistance to the alkyd varnish.

Pure poly urethanes (with no alkyd resin) are always two-part products. They cure in several ways: With the addition of moisture (an example is Gori 11a Glue), with heat (many common plastics), or they are packaged as two separate components that cure after they are mixed (similar to the way two-component epoxy adhesives work).

The two-component polyurethanes are becoming more common in the furniture industry because they perform well and have a very high solidscontent, meaningless solvent to escape into the atmosphere.

One-component, "uralkyd" polyurethane has become so dominant in the woodworking and home-consumer world that it's now becoming somewhat difficult to even find old-fashioned alkyd varnish.

Confusion has been added in the last decade or so with the introduction of water-based finishes, some of which combine polyurethane with acrylic resins. These finishes are sometimes labeled "polyurethane," with no obvious reference to their being an entirely different class of finish, one that performs less well than oil-based polyurethane and has very different application characteristics.

This isn't to say you shouldn't use water-based polyurethane. Just be aware that it is an entirely different finish - a water-based finish. This article deals solely with oil-based polyurethane.

The Myths

Myths are much more prevalent in finishing than in woodworking because finishes are chemistry, and you can't always "see" differences in chemistry. For example, polyurethane and lacquer look the same, both in a can and on the wood, even though they have very different characteristics.

In contrast, woodworking is physics. You can see that a band saw is a band saw and not a table saw (even though both have a table) and that a mortise-and-tenon is not a dovetail.

So authors and manufacturers have much more opportunity to provide inaccurate information, intentionally or not, about finishes than about woodworking tools and procedures. And consumers are more vulnerable to misinformation - that is, "myths" - about finishing than about woodworking.

Once a myth gets into print, it's common for it to be repealed endlessly until it becomes "fact," simply because everyone says