Flexner on Finishing: Sealers – What Are They?
by Bob Flexner
In the late 1980s, I hosted a local call-in radio show dealing with finishing and restoration. I remember one caller explaining that he had applied four coats of tung oil; he asked what he should use to seal the wood!
OK, so he was probably using wiping varnish falsely labeled “tung oil,” as I’ve explained many times, but four coats of any finish will seal wood quite well. Just one coat, in fact, is enough unless the finish has been excessively thinned.
Clearly, this fellow misunderstood the term “sealing.” But he’s not alone; there’s probably no term in finishing that’s more misunderstood.
A Little History
In the 19th and very early 20th centuries, there was no discussion of sealers and, so far as I can determine, there were no products marketed as sealers. The likely explanation is that almost everyone, including furniture manufacturers and painters, used shellac, which is fairly easy to sand.
But in the 1920s nitrocellulose lacquer was introduced and replaced shellac in furniture factories. Unlike shellac, lacquer is not easy to sand because it gums up sandpaper, causing small lumps called “corns.” Because the first coat should always be sanded to remove the roughness, manufacturers created an easy-to-sand finish, which they called “sanding sealer.” The name is logical enough when you remember its purpose.
Varnish has a similar problem of gumming up sandpaper, so as the market for this finish grew, sanding sealers were developed for it, also.
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