Over my career making and restoring furniture I’ve found washcoats helpful now and then when staining wood. A washcoat is a finish, any finish, thinned with two or more parts thinner and applied to wood under a stain to partially seal the wood so penetration of the stain is limited. The purpose is to reduce blotching in blotch-prone woods such as pine, cherry and birch. That is, in softwoods and tight-grained hardwoods.
Because most old furniture is made from mahogany or walnut, which don’t blotch in an ugly manner, I don’t use washcoats that often in my restoration work. But now and then on other projects, I find them useful for getting a more even coloring with stain. Washcoats are easy to make from lacquer, which I typically use. I simply thin the lacquer enough to partially seal the wood enough to block the blotching but not so much the stain doesn’t color at all.
When I started teaching and writing for amateur woodworkers, I came across the product usually called “wood conditioner.” The major brands typically instruct you to apply the stain within two hours. When I tried it, I found it didn’t work well. It took me years to realize the problem. The lacquer I was using dries rapidly, so by the time I got around to applying the stain, the lacquer was dry. The varnish or oil in wood conditioners dries very slowly, often requiring overnight. So the trick to using wood conditioners effectively is to allow overnight drying before applying a stain. But this is rarely what the directions say to do.
So why do manufacturers give you directions that don’t work? I don’t know the answer to this, but I suspect it’s because they are marketing to an audience that wants to complete a project in an afternoon, or at least in a day. Their goal is to “move product off the shelf” as many companies have said to me. They don’t care much for the customer having success with the product. As one manufacturer explained to me when I pointed out that the product doesn’t work with the directions given, “They’ll just figure they did something wrong and try again with a new can.”
If you want to know why finishing seems so difficult at times, this is the explanation. Manufacturers give you bad information. The problem applies almost entirely to the market for amateurs and small shops. It rarely applies to factories. I don’t know what to do about this other than to give you directions that do work. I’ve talked to a lot of manufacturers. My experience is they either don’t care, or they have no idea what their finishing products actually are and how they work to begin with. I’ve found no interest among them in making changes, and, in fact, none have that I know of.
How about individual brands and the differences among them in their instructions? Remember that my point is that manufacturers don’t understand their own products. They are providing directions they believe are those desired by their target audience. And these directions are all over the map even though the products are nearly the same, thinned oil or varnish. These 10 brands, some of which don’t exist anymore, can be grouped in three categories plus one.
Brands that instruct to wait a minumum time before applying a stain, 30 minutes to two hours: Varathane, Colony and the old version of Old Masters.
Brands that tell you to apply the stain within two hours: Minwax, Olympic and General Finishes.
Brands that tell you to wait from 8 to 24 hours before applying a stain: Benjamin Moore, Daly’s Benite and the new version of Old Masters.
Bix says you don’t have to wait; you can stain anytime.
To repeat, if you want to partially block stain from penetrating and causing blotching on softwoods or tight-grained hardwoods, you can apply any finish thinned with two or more parts thinner. But to be effective, you have to let this thinned finish dry before applying the stain.
All these variations in wood-conditioner instructions can’t help but cause confusion with the term, “washcoat.”
– Bob Flexner