Because a rough concrete edge in a swimming pool has constant contact with water, adding concrete flashing over the original concrete will likely peel away after a few months. The best way to smooth out a rough edge for a concrete pool is to sand the edge flat. This will help make quick work of repairing an edge that may hurt someone getting in or out of the swimming pool.
Drain the pool of its water in accordance with the pool manufacturer's instructions as well as local laws. Usually concrete pools have a built-in drainage system. If not, you'll need a waste pump to pump out the water.
Attach a metal mesh diamond disc to an angle grinder, which requires a specialized spanner wrench (supplied with the grinder) to loosen the disc-retaining nut on the cutter head. Remove the retaining nut, place the disc on the shaft and tighten the retaining nut back onto the shaft, using the spanner wrench.
Connect the angle grinder to an electrical outlet or to an air compressor if the grinder is air-driven.
Sand the rough edge with the angle grinder. Run the grinder over the rough texture in several shallow passes. Do not remove a lot of material at one time. If you cut out too much, it will require intensive work to repair the pool. Create a slightly angled edge to prevent sharp corners from becoming an issue for swimmers.
Vacuum out the pool with a wet/dry vacuum before refilling the pool with water. This will remove any cement dust from the pool.