Stair banister posts have the technical name of newl posts and serve to anchor the banister securely to the top and bottom of the staircase. Since these posts bear the majority of stress and weight, they often loosen over time. The loose post is the result of the underlying screw threads that pull out from the floor. Fixing these loose posts simply requires that you insert additional supports to tighten up the post and then disguise the repair for a professional appearance.
Insert a 3/4-inch diameter spade bit into a power drill and place the tip of the bit at the base of the loose post on the outside of the staircase. Line the bit up so that the hole will be located directly where one of the stair treads falls on the inside of the staircase.
Squeeze the trigger of the drill to create a 1/4-inch-deep depression and then release the trigger.
Replace the paddle bit with a 1/4-inch diameter extra long 10- to 12-inch wood bit and insert the bit into the center of the previous hole. Hold a square or a level above the drill and squeeze the trigger of the drill to create a 1/2-inch-deep hole in the tread. Keep the top of the drill aligned with the square to ensure the hole stays straight.
Remove the 1/4-inch diameter bit and replace it with a 1/8-inch diameter bit. Insert the bit into the hole and drill another 2 to 3 inches deep.
Insert a long 10- to 12-inch screw into the hole and tighten it using a standard power drill. The head of the screw should be under the surface of the stair post, inside the concave portion.
Scoop up 1 tbsp. of wood putty using a putty knife and press it into the top of the screw hole. Scrape off the excess putty from the top until it is flush with the post and then wait until it dries. Sand the putty with a 200-grit sandpaper and then paint it to match the stairs to disguise the repair.