CAUSES: Chemical extractives in wood react with metal flashing and produce a stain below the flashing. Stains also can occur in other areas where the wood surface is in contact with metal: outdoor faucets, air conditioners or conduits are examples.
SOLUTIONS: Chemical-extractive stains normally can be removed by washing with a mild solution of household detergent and water. Fill any cracks between the flashing and a wood surface with a top-quality acrylic or butyl rubber caulk to prevent the stain from recurring. After the surface is dry, recoat with a clear wood finish or other coating or stain.