Holes in concrete (smaller ones generally) and fibreglass pools may be filled with an epoxy filler, such as Megapoxy PM. There are other brands as well.
It’s important to follow their mixing and application instructions carefully.
Generally, you mix equal portions by knife on a clean board until homogeneous. Then knife into the holes, slightly over filling. It will be quite sticky and difficult to get a smooth finish.
To smooth out successfully, have a cup of dishwashing detergent and water (about 10 – 15 % detergent is ample). Simply dip in your fingers and then run gently over the Epoxy surface to smooth. Repeat as soon as you feel Epoxy wanting to stick to your fingers. (Wear disposable gloves)
Do not add more epoxy after doing this, as it won’t adhere to the now wet detergent surface.
You can sand if necessary once cured to remove any rough spots, though there won’t be many. Allow curing before applying paint, generally for 24 hours.
Can also be used in cleaned out non moving cracks as well.
Any left over unused epoxy will keep for several years in resealed containers and kept in cool dry place.