There are several ways to remove cat scratches from your stair banister. First, you can try a filler crayon, which is available at any hardware store. Often, this is all that it will take if the scratch isn’t deep and only goes through the surface of the varnish or stain. If it is more serious, you can combine the crayon or wax filler with a sealer, and that should restore your banister. If it is more serious, you may have to sand the area lightly to remove the scratch and use a good wax or polymer-based sealant to repair the damage and then wax the area to finish the job.