Preventing Blotching Using A Wash Coat #1: Finishing with Wash Coats

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Blog entry by pjones46 posted 01-04-2015 03:10 AM 3782 reads 5 times favorited 2 comments Add to Favorites Watch
no previous part Part 1 of Preventing Blotching Using A Wash Coat series Part 2: Waterbourne Finish Coatings »

Wash Coat #1: Finishing with Wash Coats

I am starting to put together an article covering finishing and this will be part of the coverage but not just limited to preventing blotching as a wash coat will aid in a more consistent staining color. This will be updated as my thoughts are organized.

This is only a small portion:

A wash coat is a coat of thinned finish that’s applied to bare wood to partially seal the surface before a stain is applied. It reduces the amount of stain from soaking into the wood and causing blotching.

It works well on woods like alder, aspen, birch, cherry, and pine as well as opened grained woods. The wash coat is usually made with shellac, vinyl sealer, or glue-size; however, you can use other finishes as long as the stain does not dissolve it. To avoid problems, don’t use an alcohol reduced dye with a shellac wash coat; a solvent based stain (e.g., lacquer stain) with vinyl sealer (oil-based stain is okay); or a water-soluble dye or stain with glue-size. In some cases a barrier coat application might be considered when mixing finish and stain types.

Depending on the effect you want and the type of wood and stain you’re using, you will want to vary the thickness of the wash coat. This is done by controlling the solids content of the wash coat.

Using shellac as an example, the approximate solids content (by volume, not weight) of a 2 lb. (2#) cut is 16%, a 1# cut is 10%, and a 1/2# cut is 5%. The lower the cut, the thinner each coat of shellac will be. The thinner the wash coat, the less it fills the grain and pores of the wood which allows the stain to highlight these features better or for the worse. On wood with fine grain and pores, limiting the thickness of the wash coat is very important to the final look.

Some woods are more porous than others and some stains are more likely to cause blotching. By managing the solids content of the wash coat, you can account for and control these variables.

Thick oil-base stains (gel stains) and glazes do not soak into the wood and penetrate as much so a thin (low solids) wash coat works well with them. Thin, penetrating stains soak into the wood deeper and the wash coat needs a higher solids content to keep them from blotching. But small variations in the solids content of the wash coat can make a significant difference in the appearance of the stain.

If it is a little too thick, the blotching is gone, but the grain and pores aren’t highlighted very well and the stain doesn’t add much color which may be in fact the look you want. To get the desired look, and be able to repeat it consistently, you have to deliberately control the exact solids content of the wash coat. In many cases a wash coat will reduce the intensity of the stain color to a lighter shade so testing on scrape wood is highly recommended to obtain your desired color.

To make a wash coat, start by finding out the solids content by volume, of the finish you’re using for the wash coat. The manufacturer of the finish can provide this information. Use the “solids by volume” number that the manufacturer supplies, not the “solids content by weight”. There’s a wide margin in the solids content of finishes so there’s no set rule for thinning ratios like “4:1.”

The solids content of a wash coat will usually range between 3% and 10%. To get good grain and figure definition while using a wash coat with a thick oil-base stain, the solids should be around 5%. To change a 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 glue-size.

Let me use some real numbers for an example:

Should you have some Zinsser “Seal Coat” dewaxed shellac that you use for a sealer. If you want to mix a little of it to use as a 5% wash coat, checking with the tech sheet for Zinsser Seal Coat and you will find out it’s a 2 pound cut of dewaxed shellac and has around 16% solids by volume.

Let’s say we measure out 8 ounces (1 cup) of the shellac and want to figure out how much alcohol to add to make it a 5% solids wash coat.

Divide the current solids content, 16%, by the solids content we want, 5% (16/5=3.2).

Then multiply 3.2 times the 8 ounces we have in the cup (3.2×8=25.6) which is the total resulting mix to get my 5% mix.

So we need a total of about 25.6 ounces and we already have 8 ounces of the shellac. Subtract the 8 ounces from 25.6 (25.6-8=17.6) and find out that we need to add about 17.6 more ounces of alcohol to get a 5% solids wash coat.

Adding about 17.6 ounces of alcohol to the 8 ounce cup of 2# shellac we have and that gives us a total of 25.6 ounces of shellac with about 5% solids. Our wash coat solution is ready to use.

Use of Glue-Sizing as a Blotch Control/Pre-Stain Conditioner

For years glue-sizing has been used at the beginning of the finishing process to help produce an even and smooth surface for wood as well as a wash coat to reduce blotching in woods with high figure of a mixed hard and soft wood grain. Some of these woods are cherry, pine, fir, ash, oak, walnut, and mahogany.

Glue size is also applied to the porous edges of particle and fiber boards to prevent over-absorption of glues and finishes. This helps fill the grain and give the wood more even coloration and surface quality.

Glue size is most commonly made of a watered-down, water-based, PVA adhesive. If you do plan to make your own, try something on the order of 10 parts (or a little less) water to one part white glue. Keep experimenting if this doesn’t work out right (it will vary with species).

In one of my previous articles, I used Gorilla White Wood Glue at a ratio of approximately 1 part glue to 8 parts water and in worked well as compared to a commercially produced water based pre-stained/blotch control formulation at about 25% of the cost.

Apply a glue-sizing after sanding your wood to 220 and use two coats glue-sizing as follows:

Apply a wet coat of glue-sizing allowing short time for softwood to absorb mix, wipe off excess lightly allowing the coat to dry fully 2-4 hours depending on temp and humidity. When the glue-sizing has dried, lightly sand the treated area with 220-320 grit sandpaper. This is to remove the raised grain fibers and roughness so just lightly sand.

At this point, apply the second wet coat of glue-sizing, again allowing short time for any absorption that may occur. Then as before, wipe off excess lightly allowing the coat to fully dry. Hand sand with 320 grit sandpaper along the grain as a final buff before applying stain.

Make sure you remove any dust with a clean dry 100% cotton cloth or paper towel, preferably a tack cloth and your ready to proceed to your stain or finish.

Wash Coat #1: Finishing with Wash Coats
©2010-2015 Paul Jones – All Rights Reserved

-- Respectfully, Paul

2 comments so far

View ScottKaye's profile (online now)


467 posts in 1371 days

#1 posted 01-07-2015 01:42 PM

Added this to my favorites as this is a very good read and a wealth of information I can refer back to. I recommend others do the same

-- "Nothing happens until you build it"

View pjones46's profile


986 posts in 2061 days

#2 posted 01-07-2015 02:48 PM

Thank you, just thought it should be put out there to help people so they don’t have to go through the same long learning curve. If it helps one person, it is worth the effort.

-- Respectfully, Paul

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