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This is Not Rocket Science

Jun 21, 2016 | Expert's Corner | 0 comments

Back in the 1970s and 80s, it was common to see explanations of the difference between Danish oil and wiping varnish described as that between oil and resin-fortified oil.

I couldn’t make sense of this explanation because the term “resin” is so vague. It commonly refers to fossilized tree saps such as rosin, copal and amber, and sometimes to synthetic alkyd and polyurethane. So, is one of these resins just added to the oil to “fortify” it, or is something else done?

A further confusion is that linseed oil and tung oil are, themselves, sometimes classified as resins. So, is wiping varnish oil fortified with more oil? Doesn’t make sense.

In 1990, I added some clarity to the issue in an article I wrote for Woodwork magazine. Danish oil is simply a mixture of linseed oil and alkyd or polyurethane varnish. That is, you take a can of boiled linseed oil and add some of it to a can of polyurethane and you have Danish oil.

The mixture dries very similar to boiled linseed oil alone—that is, soft, wrinkled and sticky if all the excess isn’t wiped off after each coat. But several coats are a little glossier and slightly more water resistant than linseed oil alone because of the polyurethane component.

In significant contrast, wiping varnish, which is often mislabeled “oil” or “tung oil,” is simply alkyd or polyurethane varnish thinned about half with mineral spirits (paint thinner). That is, you take a regular can of varnish or polyurethane that you buy at the store and add some paint thinner to it.

There is no oil involved except in the making of the original varnish. But that oil is chemically reacted with the alkyd or polyurethane “resin” to make the varnish. It is no longer oil, anymore than yeast is still yeast after it has been reacted with flour to make bread.

Just as unthinned alkyd and polyurethane varnish dry hard, so does the thinned varnish. Because it dries hard, you can build it up to a thicker, more protective, film on the wood with numerous coats. You don’t have to wipe off the excess to get a functional result.

The difference in the way Danish oil and wiping varnish dry is well illustrated in the photo of two dried puddles on glass.

Over the last two decades, seemingly everyone writing in woodworking books and magazines has adopted this explanation of Danish oil and wiping varnish. Even the term, “wiping varnish,” which I coined to categorize the thinned varnishes, has become part of the finishing vocabulary (though most manufacturers have continued with their confusing and sometimes misleading labeling).

I have felt very gratified at having been able to contribute to a better understanding of these two very popular finishes.

So imagine my sadness the other day when I read in a prominent woodworking magazine that the difference between Danish oil and wiping varnish is the percentage of “resin” included. According to this author, if more than 35%, it’s wiping varnish. If less, it’s Danish oil. (Try to find that information on the label!)

After two decades, here we are back at the beginning, with an explanation that makes no sense. There’s no need for this. Finishes aren’t rocket science.