Most of the hardwoods used for guitar bodies and necks - e.g. all Rosewoods, Koa, Walnut, Mahogany, and more - have very open pore structure. These open pores show as a multitude of tiny depressions throughout the smoothly sanded surface of these hardwoods. It is essential to fill these tiny depressions to achieve a flat and smooth surface. This is the most important step in the finishing process to achieving a perfect glass smooth final finish. Committing the time and keen attention toward filling the pores effectively, and thoroughly, at this stage will reduce the time and effort used in spraying and sanding the clear lacquer coats later in the process.