Hammer down any nails that are raised from the deck boards until they are flush with the floor.
Mix powdered oxygen bleach with water in a mixing bucket. Stir until the powder dissolves. Don't use chlorine bleach. Oxygen bleach is safe for the plants surrounding the deck when rinsed off.
Pour the bleach over stained areas of the deck as needed and scrub the wood with a soft-to-medium scrub brush. Don't scrub too hard, because you can damage the wood's surface. Allow the bleach to remove the stain instead of the scrubbing.
Rinse the deck with a water hose to remove the bleach solution. Allow the wood to dry after rinsing.
Pour the water sealant or cedar stain into the paint tray. Use products specifically manufactured for cedar for the best results. Dip the paintbrush into the tray and paint the rails and spindles first. Then paint the cracks between each board on the deck floor.
Roll the paint roller into the sealant or stain and roll the product across each board on the deck floor, refilling the paint tray as needed. Allow the product to dry completely, following the manufacturer's instructions. Follow with a second coat if needed.