Just remove the burrs, particularly on the inside of the spring, at the point where its cut. I use a coarse stone for smoothing it flat, sandpaper is better for wood and paint.
Taking it to a mirror smooth finish is a waste of time, just take off the burrs and get it flat. Consider what the spring is loaded against. If it's not equally finished, or if there is a washer in between, you aren't accomplishing anything with a polished spring anyway.
Get you a combination sharpening stone, coarse on one side, fine on the other. It's a necessity for most precision gun work anyway. Especially for triggers, latch rods, and hardened parts. They are pretty cheap.