Protect adjacent walls and other surfaces with painter's tape and masking paper or plastic sheeting.
Coat the stairs semi-liberally with eco-friendly stripper to remove any old finish. Apply stripper with a paintbrush and leave it to sit for the time indicated in the directions. Remove the stripper and dissolved finish with a paint scraper or putty knife. Access hard to reach areas using a mini scrub brush. Rinse the stairs with fresh water when finished. Spray outdoor stairs with a hose or use a water-drenched towel on indoor stairs.
Paint the stairs generously with wood brightener. Brightener will help restore the natural tones of the pine. Allow the brightener to sit for the time indicated in the directions, then rinse with water.
Sand the stairs starting with 120-grit sandpaper, then follow up with 220-grit sandpaper. Vacuum all sanding dust when finished.
Apply stain, if desired. Coat the stairs thickly with stain, allowing it to soak in, then wipe away the excess. Apply additional coats until the desired saturation is achieved.
Paint the stairs with wood finish such as polyurethane or urethane. Apply the number of coats indicated in the directions. Also if directed, lightly sand between coats of polyurethane with 320- to 400-grit sandpaper or #0000 steel wool.