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Furniture Finishing & RestorationHow to Prevent Blotching

In this sample, I was experimenting with different solids contents of the washcoat to find the thickness that provided the most color and grain definition with the least amount of blotching. The washcoat on the left side is lower in solids and the right side is higher solids. The thinner washcoat on the left allowed the stain to add more color. 
To make a washcoat, start by finding out the solids content, by volume, of the finish you're using for the washcoat. The manufacturer of the finish can provide this information. Don't use the "solids content by weight" number that the manufacturer supplies, you need the "solids by volume" number. The drawing on the left represents a measuring cup filled with a finish that contains 20% solids. The other 80% is solvents and thinners that will evaporate when the finish dries/cures. There's a wide margin in the solids content of finishes so there's no set rule for thinning ratios like "2:1." 
To be effective, the solids content of a washcoat will generally range between 3% and 10%. To get good grain and figure definition while using a washcoat with a thick oilbase stain, the solids should be around 5%. To change our 20% solids finish to a 5% solids finish, we need to add the right amount of the proper thinner. For shellac, the thinner is alcohol; for vinyl sealer the thinner is lacquer thinner; and water is used with glusize.
I use a quart measuring cup most of the time so I'll base the math steps on a cup that holds 32 ounces of finish. In the drawing above, we have a finish that contains 20% solids and we want to thin it until it contains 5% solids. So we divide 20% by 5% and get a result of 4 (20/5=4). Now multiply the 4 times the container size, 32 ounces, to get the total liquid volume needed for a 5% solution (4x32=128). We need a total of 128 ounces to get a 5% solution. We already have 32 ounces, so we subtract that from the total we need (12832=96). This tells us we need to add 96 ounces of thinner to change the 20% solids finish to a 5% solids finish. Since 128 ounces is a gallon, you may want to mix up a smaller batch if you don't need that much.
Let's use another example with some real numbers. I have some Zinsser "Seal Coat" dewaxed shellac that I use for a sealer (it's not a good topcoat). I want to mix a little of it to use as a 5% washcoat. I check with the tech sheet for Zinsser Seal Coat and find out it's a 2 pound cut of dewaxed shellac and has around 16% solids by volume. Measure 8 ounces (1 cup) of the shellac and figure out how much alcohol to add to make it a 5% solids washcoat. I divide the current solids content, 16%, by the solids content I want, 5% (16/5=3.2). Then I multiply 3.2 times the 8 ounces I have in the cup (3.2*8=25.6). I need a total of about 25.6 ounces and I already have 8 ounces. So I subtract the 8 ounces from 25.6 (25.68=17.6) and find out that I need to add about 17.6 more ounces of alcohol to get a 5% solids washcoat. I add about 17.6 ounces (middle cup in the figure) of alcohol to the cup of 2# shellac I have and that gives me a total of 25.6 ounces of shellac with about 5% solids (cup on the right in the figure). My washcoat solution is ready to use.
Reducing 2# Shellac (16% solids) to a 5% Washcoat
Here's the final example. The vinyl sealer I use has 11.8% solids and I want to make a 5% solids washcoat with it before applying an oilbase glaze. I don't need too much of the washcoat so I'm going to start with 8 ounces of the vinyl sealer. Go ahead and figure out the total volume of liquid you'll need. You should get 18.88 ounces (11.8/5=2.36) (2.36*8=18.88). Next, figure out how much thinner you need to add. You should get 10.88 ounces (18.888=10.88). There's the solution! Add about 10.88 ounces of lacquer thinner to the 8 ounces of vinyl sealer and you'll end up with a 5% solids washcoat.
Pad, brush, or spray a regular wet coat of the thinned finish on the bare wood (after sanding), let it dry, then sand very lightly with a very fine grit (e.g., 220, 320, or 400)to smooth the surface. Excessive sanding will remove too much of the washcoat and allow the dye or stain to soak in and cause blotching. Your application technique will affect how much material you deposit on the surface; for example, spraying will lay down more than padding. Adjust the solids content to work with your application technique.
End grain on table tops, dressers, cabinets, raised panel doors, etc. can absorb a lot more stain than the rest of the wood. That's because the end grain exposes the channels that the water that fed the wood used to flow through. To keep the end grain from getting EXTRA dark, just use a washcoat on it.
Start by sanding the end grain up to 220 grit. This will make it smoother and help limit how much it absorbs. When the end grain isn't just flat, I like to use a hand sanding pad and press it onto the wood until it conforms with the shape.
Sanding raised panel sides 
Sanding the end grain 
You can apply the washcoat just to the end grain if that's the only area that you need to control stain penetration. Don't worry if the washcoat gets on the flat surface of the wood when you apply it to the end grain... just let it dry and then sand the flat surfaces using a sander or sanding block to remove the sealer. Sand the washcoat very lightly with very fine grit paper or a sanding sponge, just enough to smooth it, and then stain the entire surface all at once.
End grain is colored evenly 
A closer look 
As always, do a LARGE sample of your finish from start to end before starting on your actual piece(s). Small samples can look deceivingly good, but on a larger surface it's a disaster! Use the bottom of a table top, shelves, or the back of doors if you don't have sufficient scrap to work with. In the sample above showing the 2 washcoats, I used the back of a door and then sanded it off before I applied the final finish. Make sure everything works the way you expected and looks the way you want it to. If you're getting blotching, increase the solids content a couple percentage points at a time until it goes away.