To achieve a smooth and even coat of paint or stain between deck boards, gather the necessary materials, such as a paint scraper, sandpaper, and brushes designed for deck surfaces. Use a crack and groove brush to reach narrow gaps and cracks, applying paint in short strokes. Fill small cracks with wood filler before painting or staining for an even finish. Protect your deck by applying the first coat of paint or stain, and use a tray to remove excess dye and avoid dripping between the boards. Allow each coat to dry completely before proceeding with additional coats.