Step 1
Sand the steps with 100-grit sandpaper. Follow by sanding with 150-grit sandpaper, then 220-grit sandpaper. Always sand in the direction of the grain on the wood. Vacuum the steps with a brush attachment to remove the sanding dust. Run a tack cloth over the steps to further remove the dust.
Step 2
Moisten a clean rag with denatured alcohol. Wipe each step with the rag.
Step 3
Apply painter’s tape along the trim of the stairs to protect the walls from the polyurethane. Cover the bottoms of any spindles or other objects on the stairs that you don’t wish to get the finish on.
Step 4
Mix two parts mineral spirits and three parts polyurethane finish into a clean bucket. Only mix enough to apply one coat to the stairs. Only the first coat of finish needs to be thinned so it penetrates the wood better.
Step 5
Dip a 2-inch natural-bristled paintbrush into the mixture. Run the brush along the brim of the bucket to remove the excess.
Step 6
Apply a thin coat of polyurethane onto the wood stairs. Start at the top stair and work your way toward the bottom. Run the brush along the stair using long, uniform strokes. Move the brush in one direction only – in the direction of the woods’ grain. Back brush over any drips or pools of polyurethane.
Step 7
Allow the first coat of finish to dry overnight. Sand the stairs smooth with 200-grit sandpaper. Vacuum the stairs, then clean with the tack cloth and denatured alcohol.
Step 8
Apply a second coat the next day in the same manner using a clean brush. Do not thin the polyurethane when applying the subsequent coats. Let the second coat dry overnight. Sand, clean and apply a third coat, if necessary. A fourth or fifth coat may be necessary to achieve the desired appearance. Be sure to sand and clean the wood in between each coat. Do not sand the final coat of finish.