Imagine the perfect latex paint. It sticks to anything, covers in one coat, levels smoothly, is strong yet flexible and lasts for decades. It never fades or chalks. It discourages mildew growth and is formulated so you can apply it in cold weather. And it doesn't drip. Unfortunately, that perfect paint has yet to be developed. Never will be, either. The ingredients in a batch of paint just have too many limitations and trade-offs. But while there may be no such thing as perfect paint, there are definitely high- and low-quality paints. Here we'll look at the differences, particularly as they apply to exterior latex paints. And why it's worth it to buy the good stuff.
Figure A: High Quality vs. Low Quality
High-quality paint has more total solids (pigments plus resins) and fewer solvents than lower-quality paint. So while the two may go on with the same thickness, when the solvents evaporate, a high-quality paint leaves a thicker, tougher paint film.
Low-quality paint has fewer solids and more solvents. Once the solvents evaporate, the thinner paint film left behind is less durable and doesn’t cover as well. You’ll spend more time and money repainting to achieve the performance of a high-quality paint.
To most of us, one opened can of paint may look pretty much like the next, but not to the chemists who formulate it. A typical batch of latex base paint—the untinted paint to which colorants are added in the store—can contain 15 to 20 ingredients or more. Base paint ingredients can be lumped into four categories:
Pigments are finely ground particles that give paint its opacity and ability to “hide” the surface it’s applied to. (Colorants are the tints added at the hardware store or home center to produce the exact color you want.)
Resins (or binders) are the ingredients that hold the pigments and colorants in place after the paint dries.
Solvents (also called carriers) are the liquids that transport the above ingredients from the brush or roller onto the wall. In latex paint the main solvent is water; in oil-based paint it’s primarily mineral spirits. These evaporate as the paint dries.
Additives are the chemicals that affect everything from mildew resistance to workability.
To understand how latex paint works, picture a bathtub full of water and Ping-Pong balls. Some Ping-Pong balls are white, representing the pigments. Others are—let’s pick a color—red, and represent the colorants. Some are clear and exceptionally sticky and represent the binders. The water they’re all bobbing in is the solvent. As the bathtub drains (that is, the paint dries), the clear Ping-Pong balls link together, trapping the white and red balls in a tight mesh. Bingo, there’s your paint film. The purity, quantity and size of all those Ping-Pong balls are what dictate paint quality.
The predominant and most expensive pigment in paint is titanium dioxide, a very pure white powder with exceptional hiding qualities. High-quality paint contains a high percentage of this pigment. Other, less expensive pigments such as clay, silica and talc are found in expensive paints but are more prevalent in cheap paints. They’re not as white and pure, and they’re poorer “hiders,” meaning you’ll need to apply more or thicker coats of paint to cover existing colors (Fig. B). You can’t really judge the quality of a paint by the amount of pigment in it; for instance, flat paints always contain more pigment than glossier paints. But you’ll certainly find better-quality pigments in expensive paints.
Resins—sometimes called binders—are the plastic-like ingredients in paint that bind together to create a film, encapsulating the pigments. The best latex resins are made of acrylic, and the best acrylic resins are made from small particles that can deeply penetrate the wood pores and “grab on” (Fig. C). That’s why high-quality binders last longer and better resist blistering, peeling and wear. At the store, you’ll usually find paint labeled two ways: “100 percent acrylic” or “vinyl acrylic.” Buy the higher-quality 100 percent acrylic paint.
Glossy paints contain more resins and less pigment than flat paint; it’s the resins that make a glossy paint glossy. But a high-quality gloss paint will have more resins than a low-quality gloss paint, and those resins will be higher quality. Since resins are the most expensive component in any paint, high-quality paint with high-quality resins is more expensive.
By now we know that paint sheen is determined by the ratio of binders to pigments: Flat paint has more pigment and less resin; glossy paint has less pigment and more resin. But paint sheen aside, high-quality paints will have more combined solids (pigments and resins) and less water than cheap paints. Our expensive gallon of paint contains as much as 45 percent solids by volume, while cheap stuff may have less than 30 percent (Fig. A). A higher percentage of solids means a thicker paint film, better hiding and greater durability. The remaining ingredient in a can of latex paint is primarily water, the cheapest ingredient of all, with the sole purpose of carrying the binder and pigment to the surface before evaporating.
All latex paints contain additives, but high-quality paints contain more or better ones that also cost more. Thickeners, which can significantly increase the cost of a gallon of paint, slightly slow down and smooth out your brush stroke so you leave an even, thick coat of paint. The high-quality latex paint now available will brush on almost as smoothly as oil paint. Surfactants help paint soak into the wood and adhere better as well as help stabilize the color and viscosity of paint. Mildewcides limit mildew growth after the paint has been applied (at least for a few years).
Other additives protect the quality of the liquid paint if it freezes a time or two, prevent the paint from foaming when mixed or help the paint flow better from brush to surface. These additives increase the cost of high-quality paint.
Even if exterior latex paint ingredients were listed on the can like the ingredients on a can of soda pop, it would be tough comparing paints and sorting the good from the bad. We could look for paints with more solids, higher-quality ingredients and better additives. But we’re do-it-yourselfers, not chemists! Paint chemists are a secretive group anyway; they do a lot of research and testing and don’t like other paint chemists knowing their paint recipes.
So most experts—both chemists and pro painters—will just tell you this: Buy paint that’s 100 percent acrylic, and buy paint that’s expensive (preferably on sale). You’ll get more for your money and labor! A high-quality exterior latex paint can last 10 years or more, compared with three to four years for a cheap paint. That makes it less expensive per square foot per year in the long run and easier to maintain.