Good surface preparation is the first step to any staining project. Even with new wood, the USDA Forest Products Laboratory has found that as little as two weeks’ exposure to weather and sunlight will damage the surface fibers and measurably decrease coating adhesion. A previously coated surface generally has some weathered surface fibers as well as loose coating (chalkiness or peeling), dirt, pollen and mildew. Obviously, these are things you don’t want to coat. So you have to remove them to get to a clean, stable surface.