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Water-based Alchemy

by Dean Sickler

What is Latex Paint?

Every painter/artisan runs across a problem once in a while with base paints and nowadays these are usually latex. Now what is latex paint exactly? I know that 99% of painters don't know the answer to that question and a large percentage of those wouldn't even care, even though the answer could solve many problems. However, if you really understand the material you are working with, chances are that you can change your formula or technique with confidence to "adjust to circumstances" if things start going haywire. Of course, the best thing is to anticipate problems so they can be avoided in the first place and that involves a pretty good understanding of the material.

I am fortunate to have close to my studio the Rohm & Haas Paint Quality Institute (PQI). John Stauffer, the resident paint expert, went out of his way in helping to clear up many misconceptions I have had about latex paints and primers. The company also has a website (www.paintquality.com) and great facilities for obtaining professional information.

Latex paint is a misnomer because there is actually NO latex in latex paint. Latex is a natural product that originally came from the Brazilian rubber tree (Hevea Brasiliensis) and is now mostly produced in SE Asia. The sap of the Hevea is the natural product from which real rubber is made and this is what is used to make latex gloves; it is also the product to which people are developing allergies. This natural latex product is not the same ingredient that goes into paint. What goes into paint are synthetic polymers that look just like natural latex but have a completely different chemical makeup and different properties than latex rubber.

So if a customer announces that they have an allergy to latex, you can answer with confidence that there is none of THAT kind of latex in your paint.

Latex and Acrylic

As decorative painters, we use latex house paint in many ways for which they are not designed. They are used under slow-drying glazes which keep them wet for long periods of time. We use them as a coloring agent for clear glazes. They are thinned down with lots of water for color washes and used full strength but applied sporadically as in dry-brushing. We sometimes even use latex paint as an opaque coating.

Because we use paint for all these uses, it behooves us to learn what gives a latex paint better water resistance, better blocking or less stickiness when dry and better adhesion when thinned down. Now here is where it can get a bit tricky. Latex paint is a general term which covers all paints that use synthetic polymers such as acrylic, vinyl acrylic (PVA), styrene acrylic, etc. as binders. This is what always used to confuse me, as the term "latex" is applied to most water-based paints, regardless whether the can says they are 100% acrylic, latex or vinyl styrene. It is only because natural latex and synthetic polymers share the property of looking milky when wet, and clear and flexible when dry, that they call this whole family of polymers "latex" in the paint US industry.

Our brothers and sisters in the UK use the general term "emulsion" instead of latex but it is the same type of paint that we term “latex” paint in USA.

100% acrylic resins cost twice as much as vinyl, and paint companies try to balance them to keep costs down. For example 20% acrylic and 80% vinyl ingredients make up a typical common interior house paint, while paints that have more acrylic in the mixture are better quality and cost more. PVA (polyvinyl acetate) is even cheaper and it is the main ingredient for white glues and most cheap paints.

Acrylic resins are better for interiors and exteriors in that they offer:

1) Better stain protection (washability)

2) Water resistance

3) Better adhesion

4) Better blocking (some people call this "strap down")

5) Resist cracking and blistering better

6) Resistance to alkali cleaners.

Now how can we can use this information as Decorative Painters? Points number 1 through 4 are the most important for decorative painters that use water-based glazes. When you have a base paint that does not stain and has good "hold out", the colored glaze will not grab on to the surface and you will have plenty of "slip time" to manipulate your glazes. A paint that has good adhesion will make for a more washable surface when the paint is used as a colorant for the clear glaze.

Have you ever thrown some water on a base paint and see it go two shades lighter? Chances are you are working with a vinyl paint with little acrylic resin in the ingredients. Is the wetted down paint soft under the fingernail and does it want to grab glaze? Again chances are you are working with a large vinyl and a low acrylic resin percentage in the paint.

Paints are made to do many things, not just offer stain resistance. Paints need good flow for brushing, the right viscosity to keep them from splattering, good opacity and other things like mildew and mold resistance. That is why there is no such thing as a "perfect" paint. They are all different and used for different purposes. It is just that the purposes we use them for in the decorative painting business have not been recognized by most of the major paint companies.

A paint with a high acrylic content will have much better water and stain resistance. Because of this stain resistance, a water-based glaze will not penetrate the base paint film and it will give you the best "open time" the glaze can offer. Experience will show you that water-based glazes work best over paints and primers that have a large acrylic resin content; this is because of the "blocking" ability of a good acrylic paint. When you put a slow-drying glaze on top of a cheaper base paint, the paint re-opens and "grabs" on to the glaze making it difficult to remove and manipulate. Some large paint manufacturers have been substituting vinyl for acrylic in interior paints for economy reasons and although regular painters may not notice less water and stain resistance right away, we as decorative artists have noticed our glazes start to "grab" the paint. This “grabbing” hadn’t been a problem before the vinyl/acrylic mix in those base paints was adjusted and it is not a problem when we are using a better quality base paint.

How can you tell if a paint has a high acrylic content? Unfortunately, you cannot go by the listing of ingredients on the side of the can. These are purposefully jumbled up for proprietary reasons and are there just to meet MSDS requirements. You cannot necessarily go by the claims of the manufacturer either because they just want to sell paint and they usually have several different products of various quality.

The two best indicators of quality paint are price and performance.

A good paint will cost more money because the ingredients cost more (acrylic resins are more expensive). Never go for economy on a base paint unless you know from experience that it will work with the glaze you use, or else you will find yourself needing to apply a clear barrier coat between the base paint and your glaze, thus increasing the cost of the job anyway. Performance is something you can only tell after you have bought the can and used it, or unless you have learned it from someone who has used the product.

I realize that there are a lot of general statements in this article and chemists do not like general statements because there are always ifs, ands, buts, codicils, caveats, what have you in everything. I like to take the approach of simplifying things if possible, so that more people can understand the issues…However, if you happen to have a degree in organic chemistry, be gentle with me.

Knowledge is power.

The Keys to Color: A Decorators Handbook for coloring Paints, Plasters and Glazes
The Keys to Color theory book Dean's new book is now available at Amazon and at the blog site here

 

 

1st Published Spring, 2002

Good Luck and please write with any questions/comments. No technical questions please. I am an artisan/painter, not a chemist or a manufacturer representative.

— Dean Sickler

Pingo Ergo Sum

ALL RIGHTS RESERVED. NO PART OF THIS DOCUMENT MAY BE REPRODUCED WITHOUT THE EXPRESS WRITTEN CONSENT OF DUNDEAN STUDIOS INC.