What grit sand paper should I use for a spring?

I take the spring ends to 600 grit. Then polish on a buffing wheel.

Start as coarse as needed if it has a major burr or if you are wanting to flatten the ends.

Like Dan, I’ll usually carry it out to 600 grit or so. Just depends on what role the spring plays.

Make sure you round over the very end edge. Remember the spring changes shape when being compressed. You absolutely do not want that spring edge to catch and start binding.

I use a small belt sander with 400 grit then follow up with a cratex polishing wheel. Before polishing I flaten the spring end if not already flat. The createx wheel makes quick work and leaves a nice slick surface.

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. 

^^ This

With my belt sander and cratex wheel, I can flatten, smooth and polish both ends of any gun spring in 5 to 10 minutes tops, I wouldn't think that is a waste of time.....

With my belt sander and cratex wheel, I can flatten, smooth and polish both ends of any gun spring in 5 to 10 minutes tops, I wouldn't think that is a waste of time.....

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Knock yourself out if you like doing it. As for myself, I have other things I need to do besides make the end of a spring shiny for no real reason. I work full time and free time is a precious commodity for me.