From contributor R:
Find out for yourself. Get a 3 or 4 foot scrap piece of wood, sand carefully, and tape off 6 - 8" sections. The first section leave the wood raw. Then apply washcoat in thinner increments on each section. Start with 10 parts thinner to 1 part lacquer and go all the way to 1:1. When dry, scothbrite lightly and wipe your stain on the whole board and topcoat. You'll then know what a washcoat is and see how different ratios can change the look to match your sample.
From contributor M:
There are some finishers that prefer to reduce their wash coat to about 10 to 15% coating and the rest of thinners. This may vary depending on the wood, if it is closed or open grain, and the purpose for using the wash coats. Naturally, because all coatings contain different amounts of solids, the reduction will be different, a wash coat should contain very little solids - just enough to put a thin layer down on the substrate.
From contributor D:
A washcoat is used for one of two reasons. Either you need to isolate one layer of coating application from another or you need to lock in an applied coat of a material, so you lay down a washcoat. Or it is used as a first coat, like a sealer, so that you can either size the wood (to control blotching of stains), or raise the grain so you can cut the whiskers, or both.
The idea of a washcoat is that it can lock in your previous coat, serve as a barrier coat, seal and/or size your substrate, yet it does all this without contributing that much to the dried film thickness of your overall coatings. The washcoat is supposed be very low in solids (otherwise it would not be a washcoat, it would be a sealer coat), so that there is not much added to your film thickness.
The difference between a washcoat and sealer coat is only in the film thickness you are giving to your finishing. Typically, washcoats are between 3% to 6% resin content by volume.
Washcoats can be made - you dilute your own coatings to make them - from your first coat of finish if your finish system is one of those so-called self-sealing types of which most finish systems used in wood coatings are. You can also use specialty sealers (like shellac, vinyl sealer, stearated sealers only if your finish schedule allows for stearated sealers, and sometimes even glue size).
If you are doing a wiping stain, laying down a washcoat will give you a less dirty look to your coloring because of the sized substrate.
Wood conditioners are "brushable" washcoats and their instructions on the cans are wrong. Instead of wiping on and wiping off, wipe on and let dry. Then scuff sand. These are used to control penetration into the wood, hence control blotching.
From contributor M:
I know that those numbers are commonly used in books and magazines for wash coats, but I doubt that 3 or 6% would do much to seal off or be a great barrier coat on most woods. I prefer more solids and less passes, but each of us finishers has his own way of doing things.
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