Clean the staircase including, all treads on the top of the steps and all risers running along the base of the steps. Use a wood cleaner that’s compatible with your hardwood type, and wash away the dirt from the top of the staircase downward. Dry the stairs after washing by patting down the steps with a piece of cloth.
Wear a pair of safety goggles, work gloves and a face mask during the stripping and varnishing process to avoid contact with toxic materials or flying wood chips and sawdust. Search each step for any protruding nails. Place a nail set onto the top of the nail and drive the nail beneath the step surface with a hammer. This will help ensure that you have a smooth surface for applying the varnish.
Cover the wall at the sides of the stair treads and risers with masking paper held in place with a strip of masking tape. This will keep the paint remover from stripping the paint from the wall as well as the steps.
Apply a layer of paint remover to the top two steps in the staircase. Use a paintbrush to cover both the stair treads and the risers. Let the paint remover sit in place for 15 minutes to penetrate the existing stair finish. Scrape the remover and finish from the step with a putty knife. Rinse the surface with clean water. Move on to the next two stairs downward, stripping the stairs two at a time until you’ve removed all the finish.
Attach a 220-grit sanding pad to a random orbital sander with a dust extraction port. Sand teach stair tread and riser smooth, from the top of the staircase to the bottom. Remove any sawdust still on the staircase with a tack cloth.
Remove and replace the masking paper and tape to keep any paint remover from the surface of the stairs.
Apply varnish to your stairs from the top downward, working two steps at a time. Use a foam applicator to brush the varnish in place, covering first the tread of the stair and then the riser. Use overlapping rows when placing the varnish to avoid creating lap lines. Wipe away excess varnish with a piece of cloth. Allow the varnish to dry for the length of time suggested by the varnish manufacturer.
Examine the varnish color. If you’d like greater color depth, apply a second layer of varnish over the first.
Protect the varnish on your stairs with polyurethane finish. Brush the polyurethane finish over the dried varnish using a paintbrush. Follow the grain of the wood when applying the finish. Wait 72 hours for the finish to dry and remove the masking paper and tape.