# Preventing Blotching Using A Wash Coat



## pjones46 (Mar 1, 2011)

*Finishing with Wash Coats*

*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


----------



## ScottKaye (Jan 19, 2013)

pjones46 said:


> *Finishing with Wash Coats*
> 
> *Wash Coat #1: Finishing with Wash Coats*
> 
> ...


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


----------



## pjones46 (Mar 1, 2011)

pjones46 said:


> *Finishing with Wash Coats*
> 
> *Wash Coat #1: Finishing with Wash Coats*
> 
> ...


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.


----------



## pjones46 (Mar 1, 2011)

*Waterbourne Finish Coatings*

*Wash Coat #2: Waterborne Finish Coatings*

As previously mentioned in Preventing Blotching Using A Wash Coat #1, most any standard finish can be used as a wash coat. These are Lacquer (both waterborne and solvent type/nitrocellulose), polyurethane (both waterborne and oil based), Oil-based Varnish, and Shellac.

The above being said, lets talk Waterbourne. It really makes no difference which you use waterborne lacquer, waterborne shellac or waterborne polyurethane since they all are simply water-borne acrylics-none are really lacquer, shellac or polyurethane. These are simply terms created in the marketing department intended to stimulate sales.

*In Understanding Wood Finishes, Bob Flexner puts it this way:*"

"(Water-borne finishes are) often called lacquer, shellac or varnish for marketing reasons. It makes an entirely new type of finish seem familiar. Water based is often called polyurethane for the same reasons when some polyurethane resin is blended with the usual acrylic resin. This interchangeability of names adds to the confusion about finishes. When you hear or read that someone varnished a table, it could mean he or she applied either of the evaporating finishes (shellac or lacquer), a reactive finish (varnish), or a coalescing finish (water base)."

With nothing but the highest respect for Bob Flexner, I am a bit less charitable (his book is very high on my recommended list when it comes to explaining finishes, what they are (and aren't), and how they work). I believe there may have been a time when the "name game" was intended to foster a degree of familiarity. However, water-borne finishes are no longer new, and their basic formulation has not changes since they were first introduced. The "creative naming" taking place today is done with the plain and simple intention of misleading-and, it works.

While it is true that the Minwax product (and similarly labeled polyacrylics) contain a small amount of urethane resin, the dominant resin in all water-borne finishes is acrylic. There is virtually nothing about these so called "water-based polyurethane" finishes that can be compared to oil-based urethane resin varnish.

Therefore, what you have read about the negative attributes of poly applies only in the most peripheral sense to water-borne acrylics with urethane resin added.

The only major similarity is that (acrylics) *should not be applied over shellac that contains wax.*

In the context to a question will "lacquers do the job" we are, in effect, wondering if a water-borne acrylic from a different can and sold under a different name do any better. Again, our "lacquer" isn't lacquer unless we adopt a definition of lacquer that is so broad that it can be wrapped around any liquid finish that dries or cures to form a finish film. (If we do that, water-borne poly will also be lacquer.) In this instance the manufacturer uses the term "pre-catalyzed" in an effort to link its product with the true pre and post catalyzed lacquers.

Again, this is nothing but marketing subterfuge. Just as we are also treated to "Tung Oil Finishes" that contain nary a drop of tung oil; and to TV woodworkers who claim to use some magic oil when in reality they are simply applying wipe-on poly-whatever-thane. In the world of finishes, marketing claims and fact tend not to reside together. In all likelihood, the manufacturer has simply added a "hardener" to the finish. This is a chemical that creates a sort of "cross-linking" reaction within the finish for the purpose of making the cured film more durable in the face of heat, solvents, acids, alkalis, water, and water-vapor (the typical water-borne finish does not do nearly as well in the face of these hazards as an oil-based finish). This hardener is frequently added to water-borne finishes that include KCMA finish durability success in their advertising. The downside is that this hardener is very toxic, thus eliminating one of the benefits of using a water-borne in the first place. Furthermore, the finish film is still inferior to oil-based varnish in the face of these hazards.

Finally, it is not my point that there is anything "wrong "with any of the finishes that you propose to use. They are what they are and they are defined by their own set of advantages and disadvantages dependent on the proposed application.

But, to my point about the similarity (virtual sameness) of water-borne finishes; all water-borne finishes share three common ingredients that make up nearly 100% of the contents. Further, the proportion of these primary ingredients tends to be remarkable uniform from brand to brand. These components are 1) water (typically 50% to 60%), 2) Acrylic Resin (usually somewhere between 20% and 30%), and 3) Glycol Ether (about 10%). 
In those water-borne finishes that are advertized as "water-based poly" the acrylic resin is typically reduced on the order of 5% to 7% to make room for an equal amount of "polyurethane". Other chemicals are added in significantly lesser amounts for the purpose of improving various properties. For example, flow-out in water-borne products formulated to be padded or brushed; and, to reduce foaming in those designed to be sprayed.

In other words, you will get the same result with Target, General Finishes, M.L. Campbell, Mohawk, etc. My point is that the using one or the other, for whatever reason, will not pose a problem in terms of the ability to provide a finish offering equal properties and capabilities. There is a whole world of functionally equivalent water-borne finishes no matter what label is on the can.

Wash Coat #2: Waterborne Finish Coatings
©2010-2015 Paul Jones - All Rights Reserved


----------



## chopnhack (Nov 5, 2011)

pjones46 said:


> *Waterbourne Finish Coatings*
> 
> *Wash Coat #2: Waterborne Finish Coatings*
> 
> ...


Well done PJ! great write up, thanks for taking the time to do that


----------

