In this video

Before applying any primer or stain, it's essential to sand all the filled areas smooth. The presenter uses a Ryobi multi-oscillating tool with a sander feature, which is particularly effective for getting into tight corners and along fine edges where the wood filler was applied. After smoothing these detailed areas, she uses a larger, actual sander to do a general pass over all the surfaces of the stairs, ensuring an overall uniform and smooth finish. This prepares the wood perfectly for the next step, which is staining (to be covered in a separate video).