Flexner on Finishing: Wipe-on Finishes
By Bob Flexner
It’s probably fair to say that a majority, or at least a large minority, of woodworkers use a finish they can wipe on and off the wood. No expensive spray gun; not even any brush cleanup. Simple.
At least the application is simple. But these finishes have been made the most complex and confusing of all finishes by manufacturers striving for an edge (they want to convince you they have something special) and writers who either buy into the marketing or simply don’t know what they are talking about.
Wipe-on Finish Basics
There are four primary types of wipe-on finishes:
• Oil (boiled linseed and tung)
• Wiping varnish (alkyd or polyurethane varnish thinned about half with mineral spirits)
• Blends of oil and varnish (thinned or not)
• Gel varnish (alkyd or polyurethane varnish in a “gel” consistency).
Each of these finishes can be wiped or brushed, or even sprayed, on the wood and then wiped off.
Oils and blends of oil and varnish don’t harden well, so all of the excess has to be wiped off or the surface will remain sticky.
Wiping varnish can be wiped off, or it can be left damp or wet on the wood to build faster because it dries hard.
Gel varnish also dries hard but you can’t leave a thick layer without getting streaks or brush marks, so it’s better to wipe off the excess.
All of these finishes can be combined (and also thinned with mineral spirits) in any proportion, with these caveats: Adding oil to varnish means the finish can’t dry hard, so all the excess has to be wiped off, and adding any of the liquid finishes (or thinner) to gel varnish reduces the gel quality of the product.
I think this is pretty simple. But all kinds of problems are introduced by inaccurate labeling, marketing and descriptions. I’m going to discuss three: Labeling a thinned varnish “tung oil,” claiming that the product polymerizes and calling varnish “resin.”
Tung oil is harvested from the nut of the tung tree, which is native to China but is now grown in other parts of the world. It’s more water-resistant than linseed oil and replaced linseed oil around the turn of the 20th century as the oil ingredient in exterior varnishes.
These varnishes, and all varnishes at the time, were made by cooking one of these oils with a natural resin, such as amber, copal or kauri, which are all fossilized sap from pine trees.
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From the February 2013 issue #202
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