Simple in theory, not in the real world.
by Bob Flexner
A reader of my blog (at popularwoodworking.com) asked me to explain what polymerized oil is. So here goes. On one level the explanation is incredibly simple and on another it’s representative of all that makes finishing and finishes so hard to understand. Let’s begin with a definition of polymerization.
It is nothing more than the way certain finishes, including oils and varnishes, cure – that is, change from a liquid to a soft or hard solid. Oils cure soft; varnishes cure hard. Polymerizing is simply crosslinking. A whole bunch of monomers, which are single molecules, attach themselves to each other chemically to form polymers. That’s polymerization.
The most common way this happens with oils and varnishes is by oxidation. Oxygen enters the finish and causes the molecules to hook up, or crosslink. Metallic driers, which are often sold separately as “Japan drier,” act as catalysts to speed the drying. Boiled linseed oil and all varnishes, including polyurethane varnish, have driers added by the manufacturer. Raw linseed oil doesn’t have driers added, so this oil takes a very long time to cure – often months, even with all the excess removed.
Tung oil, the other common drying oil besides linseed oil, doesn’t have driers added by the manufacturer. But this oil dries rapidly enough so that it can function as a wood finish. The caveat here is that you have to be careful if what you want is really tung oil. Most of the products labeled tung oil are actually varnish thinned about half with mineral spirits (paint thinner). Instead of drying soft and slowly, they dry hard and much more rapidly. If a thinner is listed on a container labeled “tung oil,” the product is thinned varnish.
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