I am new to the finishing game, and was wondering, what is a wash coat? Is it something I can buy, or make myself, and what is its purpose?
From contributor J:
Also called a "piss-coat," it's a preliminary seal coat to raise the grain for sanding, and/or prep for stain. Some will use thinned sealer or lacquer (I use a 50-50 mix). Others will simply "fog on" a very light coat of full bodied material. It's helpful on woods that tend to stain unevenly (blotchy).
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.