Get some cloth buffing wheels, spiral sewn or concentric sewn, and some bars of polishing compound.
Use the buffing wheels on your grinder and load one color (green, gray or white for stainless steel. White (or green depending on what brand) being the finer and mirror polishing compound) on one buffing wheel and change wheels when you change colors. Use looser sewn wheels for finer polishing.
Google polishing and buffing stainless steel and you should get plenty of sites where you can get your supplies.
I've done stainless Colt 45's and Ruger Blackhawks and, yes, they shine like a mirror.