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If you go into a paint store and ask how to protect wood from the weather, you'll probably hear this answer: Use an oil-based primer, and top it with one or two coats of premium latex paint. It's widely believed that oil-based primers give the best protection from moisture, because they penetrate the wood more easily. But testing at the U.S. Department of Agriculture's Forest Products Laboratory (FPL), in Madison, Wis., shows that this conventional wisdom may be wrong. FPL scientists say that, while oil primers and paints adhere well to wood and create the best shield against rain and humidity, the resins in oilbased finishes lose their flexibility over time. No matter how well the surface is sealed, the wood moves in response to humidity and