Stainless will polish well with a good buffing wheel and proper polish. It will never look like chrome but it well be pretty close to nickel.
FWIW-the local Coast Guard boat has stainless rails. They have lots of man power to provide but they started out wet sanding with 100-grit and worked down to 400+, then started buffing and the railings are amazing. It is all in how much time you want to spend. My boat railing where made from stainless schedule 80 pipe. I went with the unpolished pipe instead of the highly polish tube because I wanted to make sure it was going to keep me on board. I then used Scoth-brite hook and loop discs on a 4-inch angle grinder and started working my way down from coarse to super-fine. My rails don't look as good as the Coasties but still look OK. The Scotch-Brite discs are pretty aggressive, even when you work down to super-fine so you would want to be careful taking them after your fenders. But they will take out the scars. If you are using them on a 4-inch angle grinder, they will be spinning at 10k RPM, so you can actually start turning your SS blue with heat. If you take that route, be careful. Don't press very hard and keep the disc moving. You will still need to buff.
Russ