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Lay out all the cut and drilled treated lumber pieces on a protected surface (e.g., covered with a drop cloth). Using a paintbrush or roller, apply your chosen outdoor wood finish, such as a stain, sealer, or exterior-grade varnish. Ensure even coverage on all surfaces, including end grain and inside drilled holes, as this is crucial for the longevity of treated lumber outdoors. Allow sufficient drying time between coats and before proceeding to assembly, as per the product instructions. Wear gloves to protect your hands.