Stair parts can be difficult to install without leaving gaps. And even if the installation is perfect, parts settle and gaps show up anyway. Stairs undergo stress from weight, expansion and contraction. Components of stairs should fit together tight, but it's not always possible. The horizontal parts of steps, also known as treads, can noticeably pull away from the wall area. Closing such gaps takes more than caulk or putty -- which won't stay put anyway. Fix it like a professional by closing those unsightly gaps with quarter-round trim.
Measure the depth of the step or tread. Measure from the vertical board at the back to the front of the step and subtract 1/4 inch if the front of the step is square. If the front of the step is rounded, measure from the vertical board in back to the spot on the tread just before it starts to curve downward.
Cut a piece of 3/8-by-3/8-inch quarter-round molding at the measurement using a miter saw.
Sand the end of the quarter-round molding using a hand-sanding block with 100-grit sandpaper. Roll the block over the end of the piece to round it off, blending it with the profile of the trim into a pleasing, round shape.
Apply stain and a clear finish to the trim and allow it to dry.
Place the quarter round in the corner, covering the gap. The rounded edge should be facing out. Quarter round has a 90-degree back that fits tight into corners. Hold it tight against against the step and against the vertical riser in back with one hand.
Shoot four 1 1/4-inch pin nails through the round top of the molding, pinning it down tight to the tread.
Fill the nail holes with a matching putty crayon. Wipe off the crayon residue with a soft cloth.