To test the paints and stains that go on the exterior of your house, we coat multiple panels of pine siding, starting with primed wood for paint and untreated wood for stains. We place the coated panels outside our Yonkers, N.Y., headquarters on an angled rack, facing south. Then we wait.

For up to three years we wait and watch, while the panels weather. In our tests, one year of angled southern exposure is equivalent to about three years on a vertical surface – such as the siding on your home. Thus, after one year of testing, we can say how well a paint or stain should do after three “normal” years. Two years of testing approximates six normal years, while three years translates to nine.

Here’s some of what our tests revealed:

¢ Exterior paint generally outlasts stains.

¢ Opaque or solid-color stains maintain their appearance longer than semitransparent stains, although the latter lets the wood grain show.

¢ With paint or stains, two coats provide significantly more protection than one. Premium grades, moreover, are usually worth the extra cost.

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¢ Paint gloss doesn’t affect performance. Pick a paint in the finish you prefer, or match the finish to the surface – flat for siding, for example or semigloss for trim.

Some products, we discovered, fail too fast, deteriorating after the equivalent of six or fewer years of normal exposure. Among paints, Glidden’s Evermore flat, although rated “very good” after three years’ normal weathering, peeled badly after six. True Value’s WeatherAll semigloss and McCloskey’s MultiUse satin also faltered after six normal years. Most of the stains that couldn’t go the distance in our tests were semitransparent products. Home Depot’s Behr Deck Plus Deck Fence & Siding; Olympic’s Wood Protector Deck, Fence & Siding 792; True Value’s Woodsman; and Sherwin-Williams’ Woodscapes all scored “fair” to “poor” for cracking after just three years’ normal weathering.

Best stains for the long term are solid products, including top-rated Olympic Premium 596 ($21) and Behr Plus 10 ($18, from Home Depot). Both retained a good appearance after nine years of normal exposure and both helped prevent mildew growth – a consideration if your house is shaded or in a damp climate.

As for paints, high-scoring flat finishes include California Fresh Coat Velvet ($27 per gallon, where available), Pratt & Lambert Accolade Eggshell ($34) and Glidden Spred-Dura (at $15, it’s a CR Best Buy). All looked at least “very good” after nine years and each proved superior at resisting dirt, color change and mildew. If you want a glossier paint with staying power, consider these Glidden products: Endurance ($32, though not readily available), a semigloss that retained an excellent appearance after nine years; low-luster Endurance Satin ($30); glossy Spred-Dura (at $20, it’s a CR Best Buy); and low-luster Spred-Dura Satin (at $19, it’s also a CR Best Buy).