A. Paul Fisette, a wood technologist at the University of Massachusetts, responds: The heartwood of western red cedar (and other species like redwood and cypress) contains dark- colored water-soluble chemical extractives. When the heartwood gets wet, these extractives dissolve, and the solution can run onto the surface of the siding. When the water evaporates, the face of the siding is left stained with brown streaks -- extractive bleed.
To avoid this, you have to prevent moisture from reaching the heartwood. The best way to minimize the effect is to pretreat all surfaces of the siding with a water repellent before installation, then treat the face with a semitransparent stain.
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