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Once the stain has completely dried (refer to product instructions for recommended drying times, usually a few hours), it's time to apply the polyurethane sealer. Sealer is extremely important for high-traffic areas like stair treads as it provides durability and protection against wear and tear. Using a clean paintbrush, apply the polyurethane in the direction of the wood grain. Similar to staining, use cardboard to protect the adjacent white risers and walls, ensuring a clean application line. The biggest tip when working with sealer is to make sure to brush out any drip marks or excess product immediately to avoid visible streaks and uneven finish. Apply multiple thin coats, allowing adequate drying time between each coat, according to the manufacturer's instructions, for maximum durability.