HI Peter
Years ago, I used to use Solvol Autosol paste (it came in a tube)to polish pistons.
I mounted the piston on a wooden dowel which I placed in a drill, and applied the Autosol with cardboard, finishing off with a soft cloth.
It worked for me, but it was 15 years ago and there might be a better way of doing it now.
Regards
Bob