Using grain filler is one of the easiest ways to get a very smooth finish on wood. Grain filler is a paste that fills the open pores and grain in a piece of wood. You apply the grain filler across the wood surface with a card or scraper. It normally takes several coats of grain filler to completely fill the pours. If you want a glassy smooth surface, like on some pianos and guitars, you will need to also do several additional steps applying finish coats, wet sanding and buffing.