You can use a wire brush as pointed out above, just don't use it on the skirts or the ring belt. The rings have to seal to the piston to, and a wire brush will mar the ring grooves and kill your ring seal.
Brass brush used on the sides, the mild steel on top. Re smoothie if need be. The sides can be a bug PIA only beat by cleaning the piston ring area. Cars should be taken not to mar or gouge that area. I don’t care if you use a screw driver, take care in keeping that area perfect.
