Most homeowners refinish their siding every six to 10 years. To find out which paints and stains will probably last that long on your house, we begin by spreading those finishes over untreated yellow-pine boards. Those boards tend to swell and crack, creating an especially tough test. Then we face the boards south at a 45-degree angle from vertical on the roof of our Yonkers, N.Y., headquarters. That intensifies the effects of sun and the elements on paints and stains, allowing each year of outdoor testing to equal roughly three years on your home. Here's what we check for:

Dirt buildup. Airborne particles cling to paint and stain after several years, promoting mildew. As the Ratings show, many paints looked dirty after the equivalent of three years on a house, and all of them appeared soiled after the equivalent of nine years. Cleaning the dirt involves scrubbing or pressure-washing.

Color change. Over time, sunlight causes paint and stain to fade, change color, or develop a chalky film on its surface. Some paints changed color after only a year of testing. The only alternative to tolerating it is refinishing.

Mildew. Stains and spotting from mildew are common on northern exposures and in damp, shaded areas. Removal involves scrubbing paints and stains with a solution of bleach and water.

Cracking. All of our tested paints resisted cracking. But most semitransparent stains allowed our pine boards to crack more readily than with opaque stains. If you see cracks on your siding, we recommend refinishing before they worsen and cause more damage.