Flexner on Finishing: Sealers & Washcoats
By Bob Flexner
It would be difficult to find wood finishing subjects that have been made more confusing than sealing and washcoating. This is unfortunate because these procedures are very simple and easy to understand.
A sealer is the first coat of finish you apply to the wood. It enters the pores, dries and stops them up so liquids don’t penetrate easily. It “seals” the wood. The sealer can be the finish itself (any finish), or it can be a special product designed to solve a problem.
A washcoat is any finish thinned to 10 percent-or-less solids content and used to partially stop up the pores in the wood (so a stain will still add some color), or provide a thinner barrier between color coats (stain, glaze, filler or toner) to limit the total finish build. The commercial varnish product labeled “wood conditioner” is a washcoat.
The steps for getting good results with any finish are as follows:
1. After preparing the wood, apply the first coat – by definition, the sealer coat.
2. Let the coat dry.
3. Sand this coat smooth so subsequent coats will be smooth (the first coat is always a little rough).
4. Remove dust and apply the next coat.
5. Apply as many more coats as you want. Sand between these coats if there are problems or roughness you want to remove.
Use the finish itself for the first coat unless you want to avoid one of two problems: the finish you’re using for final coats clogs the sandpaper, or there’s something in the wood that you want to block off. Use sanding sealer to avoid the clogging. Use shellac to block off problems.
Oil-based polyurethane and all water-based finishes sand and powder easily without clogging sandpaper, but varnish and nitrocellulose lacquer gum up sandpaper. So manufacturers provide a special product to be used under varnish and lacquer, properly called “sanding sealer,” but sometimes misleadingly labeled “sealer.”
The purpose of sanding sealer is simply to speed production. It takes significantly less effort (and less sandpaper) to sand large surfaces.
But sanding sealer reduces the durability of the total finish build because it doesn’t dry as hard or as water resistant as the finish itself, and subsequent coats of finish don’t bond as well to the sealer as they do to the wood or to previous coats of finish. So more is lost than gained by using sanding sealer on smaller objects.
Article: Read Bob Flexner’s article on finishing cherry, free on our web site.
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From the December 2012 issue #201
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