Go too walmart, they have instablue in the hunting section.
go too a local automotive paint supply store, they have wet/dry paper, all the way up too 2000 grit.
Buy several grits, from 600, 1000, 1200, 1600, 2000.
Disassemble, and remove all finish in graduates from coarsest, too finest.
As you do this, you are making scratches, but the finer you go, the less noticable they are.
By the time you get too 2000 grit, it will be mirror finished.
Completely clean all metal surfaces with break cleaner, too remove any oils left on surface.
Wear nitrile gloves while doing this, because your skin has oils in it.
Use a dry rag, soak the rag in the instablue liberally, and rub the surfaces until darkened.
Repeat 5-9 times with a water rinse inbetween each application.
Your end product, will reflect your efforts.
Do not take short cuts, and do not use sttel wool as dirrected in the instablue bottle.
Good luck, my 870 police folder came out great !!