Some older concrete and rendered pools may have areas that are drummy (hollow sound when you tap it with a hammer), or areas that are worn away. They can be repaired using a sand / cement mix such as Selly’s Quickcrete or any similar material suitable for water immersion. (Not just weather resistant). For smaller areas you may prefer to use an epoxy filler. Which ever method is used make sure they are fully cured before applying Epotec. If you leave the surface unfilled it will be rough and uncomfortable to the touch.