Rub with aluminum foil, Google for suggestions on what to mix it with, water, cider vinegar etc.
If you are using the bike on technical trails and single-track a lot, you might want to consider a new set of handlebars. If there's rust on the outside, then it could be even worse on the inside of the bars. That might lead to a catastrophic failure when you are jumping over a rock or something.
I don't do any true mountain biking, I just use it on the road for leisure/fitness (I put the thread in this forum because the bike itself is considered a "mountain bike".... or so I was told)
Sorry, just a newbie
Specialized Roubaix - Canfield Brothers Yelli Screamy - GT Karakoram SS - Soma Double Cross Disc
aluminum foil works and so does a can of never dull
If your bike isnt getting dirty, then you arent doing it right.