When you choose exterior paint, you have a wider range of options than you do when choosing interior paints for two reasons. One is that exterior paint must protect against a variety of conditions that aren't present indoors, and the other is that most states don't prohibit VOCs in exterior paint, although they do limit them. Oil paints have their uses, but if you're looking for the best-quality exterior paint for wood siding and trim, you probably can't do better than a 100 percent acrylic latex product. It's durable, it remains flexible and it resists fading.
Oil- and Water-Based House Paints
Exterior house paints consist of a mixture of resins mixed with pigments and carried in a vehicle, such as a petroleum distillate or water. The ratio of solid material to solvent, as well as the chemical characteristics of the solids, determines the degree of protection the coating provides and how long it lasts.
- Traditional oil paints consist of a polymerizing oil -- usually linseed oil -- carried in a petroleum solvent, such as turpentine or mineral spirits. The oil gradually hardens when exposed to the atmosphere -- a process called curing. The paint usually contains a natural or synthetic resin that speeds the hardening process and produces a more durable film.
- Latex house paints are formulated by suspending natural and synthetic resins in a water-soluble emulsion, and mixing the emulsion with water. When the water evaporates, the emulsion reacts with the atmosphere, and the film cures. Acrylic latex paint contains an acrylic resin to toughen the film and make it last longer.
Comparing Latex and Oil
Most oil paints are formulated to conform within the limits that many states place on VOCs -- volatile organic compounds. For flat paints, the usual limit is 250 grams per liter, and for gloss paints, it's 380 grams per liter.
This means that air quality isn't the main determinant when deciding whether to use an oil- or water-based product. Instead, the quality of the film is more important, and because it's made from rubber, an acrylic latex film is able to move with the wood, so it lasts longer.
One hundred percent acrylic products are formulated with plastic resins to be harder than traditional latex paints, so they last even longer and provide superior protection. Oil paints tend to dry harder and glossier, though, so if that's the look you're after, you may prefer an oil-based product.
Gloss or Flat?
The sheen of a paint product is a factor of the ratio of pigment the paint film contains compared to all the other solids -- the higher this ratio, which is known as the pigment volume concentration -- the flatter the paint. Pigments are the least durable of all the solid materials, so a flat paint with a high PVC is generally less durable than a gloss paint. On the other hand, pigments reflect light, so a high PVC paint provides better protection from the harmful effects of ultraviolet sunlight.
Semi-gloss and gloss paints are best for shady locations; if the surface you're painting is in full sun, use flat paint.
Although there are four broad categories of paint, two aren't appropriate for most house painting projects.
- Evaporative paints, such as lacquer or shellac, leave a hard surface when all the solvent has evaporated, but the surface doesn't undergo any other chemical changes and isn't durable enough for exterior siding or trim.
- Catalytic paints, such as automotive polyurethane, are just the opposite -- they are very durable, but they are also noxious and expensive. Moreover, applying one requires special equipment.
What to Look For
If you're looking for the best paint quality, be prepared to pay for it. Most paint companies offer economy as well as premium products, and the premium products contain better ingredients.
To get the best paint, look for a 100 percent acrylic latex product with a high solids content. Don't try to skip a step by applying a paint/primer mixture -- you'll probably end up having to apply an extra coat. Apply a high-solids, stain-blocking wood primer to bare wood and follow this with two coats of quality house paint or trim enamel.