Are you talking about doing the job yourself or having a professional gunsmith do it?
Either way in order to remove slight pitting, the best method is to bead or sand blast it with a blaster. Then depending on if you want a matte finish or a polished blued finish you could either leave it rough and blue it or polish it down to a smoth finish and hot blue it.
Unless it's an old traditional looking sporter, personally if I were doing this with a gun I owned I'd have all the steel and other metal parts sand blasted to a satin matte surface then have it manganese parkerized on the steel to a very dark (almost black) charcoal gray, and then any aluminum alloy parts annodized black, if any. I find polished hot blueing to be great on older traditional guns or any gun if you live in a very dry climate, but I find phosphate parkerizing to be a much better and a more utilitarian finish for a pretty inexpensive price. If you want even better corrosion resistance you can have a baked-on or heat cured epoxy matte paint applied on top of the phosphate parkerized finish. The British military has utilized this method for decades on it's arsenal rebuilt guns and have had great results with it's durability and corrosion resistance.
Personally I've always though that traditional blueing is a pretty useless and worthless finish on gun steel, just my opinion. Though I admit that there are a great many blued guns still around with their original blued finish that are still in relatively good condition that have survived use in the old west and even the trenches of WWI just fine.
There it sits. Alone and forgotten in a desk drawer or maybe in the bottom of your hunting pack. The lowly compass. Primarily initially replaced by the hand held GPS and now even by the new “smart” cell phones that include GPS, electronic compass – even real time imaging on aerial maps! Once the friend of every hunter and now the companion of few. It is not glamorous or glitzy, that is for sure… no bells and no whistles. Aaaaah, but let’s not rush to forget our...