A fresh coat of paint or stain on the siding and trim adds more than just curb appeal.

It also provides an important layer of protection against moisture, mildew, and the effects of the sun. Yet if you're among the roughly 85 percent of homeowners who use the same brand of house paint over and over again, you could be selling your home short.

Frequent reformulations of paints and stains mean that those that held up well the last time might not make the grade this time around. That's why we remove from our Ratings any product that has been reformulated and is not in stores, and begin testing again with the new version.

WHAT’S AVAILABLE

Major brands include Ace, Behr (sold at Home Depot), Benjamin Moore, Cabot, Dutch Boy, Glidden, Olympic, Sears, Sherwin-Williams, True Value, and Valspar (sold at Lowe’s). Exterior paints include a variety of sheens. The dullest is flat, followed by low-luster (often called eggshell or satin), semigloss, and gloss. Flatter finishes are best for siding, with the lowest-sheen best for masking imperfections. Glossy paint is most often used for trim because it highlights the details of the woodwork and is easy to clean. Stains typically come in opaque and semitransparent. Price: $15 to $49 a gallon.

HOW TO CHOOSE

Our tests of exterior paints are severe, exposing painted panels on outdoor racks angled to catch the maximum amount of sun. One year of testing is equal to approximately three years of real exposure. We examine each product’s ability to resist cracking, color change, and dirt and mildew buildup. Generally, most paints will look good for at least three years, some should look good for about six, and top-rated products should last about nine years. To choose the best paint or stain:

Buy the best paint. Our tests have found that paint grade matters. "Good" or "economy" grades don’t weather as well as top-of-the-line products. Cheaper grades also cost you more over time, since you’ll need to repaint more often.

Don’t expect stains to last as long. Opaque stains continue to outlast semitransparent stains overall. While several opaques still looked good after the equivalent of six years, many semitransparents failed after the equivalent of three years. Yet neither held up as well as the best paints. One plus: Stains, especially semitransparents, require much less surface preparation.

Consider where you live. Paints of any color accumulate dirt over time. Top-rated paints tended to resist dirt better than others, and darker colors hide it better. Good dirt resistance is important in urban areas, while mildew can be a problem in damp areas and on houses that get more shade than sun. What’s more, bright sun can change even the best-quality pigments, especially blues and yellows.

Prep properly. Scrape, sand, and clean siding thoroughly before applying paint. Plan to apply two coats. And remember that painting over other materials may require different steps. Stucco and masonry may need presealing. Vinyl siding can fade and become brittle. If you’re sanding or scraping a house built before 1978, be warned: The older coats of paint may contain lead, so you’ll need to take extra precautions.

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