Clean old redwood by brushing a solution of trisodium phosphate (TSP) and water, mixed as directed on the TSP box. Scrub with a nylon-bristle brush, focusing on stained spots but washing the entire item. Rinse off the TSP solution with a water hose and let the item dry completely.
Sand down any rough spots on the surface of your redwood with 220-grit sandpaper, unless you want a coarse finish, such as on a fence or on redwood siding. Most redwood furniture and handrails should be smooth.
Brush on a stain-blocking wood sealer. Also called a sanding sealer, this liquid is absorbed into the grain of the redwood and prevents the wood's natural resin from seeping out and discoloring the newly stained item. Apply as directed on the container and allow the solution to soak into the wood and dry.
Apply redwood stain with a paintbrush or a stain applicator pad. Use long strokes in the same direction as the wood grain. Unlike other stains, redwood stain is semiopaque and you will not wipe it off. Coat the entire item with one application.
Repeat the staining process if you want more coverage after the first application of stain dries to the touch.