When you have filled and cut the repair it is ready to be sanded. Sanding the repair is important as it ensures that the repair accepts the surface treatment you wish to use for aftertreatment. It does not matter whether it is oil, stain, lacquer, paint - you can use any surface treatment you want, as long as you sand the repair first. The best technique is to sand the wood 90% first, before you start to repair the wood. When you have filled the repair with knot filler and cut off excess knot filler, you finish the last 10% of the sanding process. Make sure to use a good flow when you sand the wood and the repair, in order to let the repair "breather" while being sanded.